I've been using a dual-screen/memento Uxie for a while, and I think it actually illustrates momentum quite well, against offensive teams at least.
What will happen is that I'll usually manage to get dual-screens up by good prediction, then memento my way out of there. Offensive pokemon are reduced to 1/4 of regular damage, allowing me to send out one of my slightly bulky set-up sweepers (Haxorus, Chandelure, Sciz), and get at least one CM or DD or SD up whether they switch out or (foolishly) decide to attack.
I think that this is a good illustration of momentum because in one turn, Uxie
-Allowed my Pokemon to come in safely
-Forced the opponent to waste a turn by almost negating its offensive stats.
It basically accomplished two objectives in a single turn, which, in conjunction, allowed me to become a threat. The opponent is now behind in terms of momentum, and is trying to end my sweep and playing defensively, rather than concentrating on trying to sweep.
If you'll bear with me for a second (this will sound a bit far-fetched), I believe that the concept of momentum shift in a battle can be summed up sort of like so. Let's say each of the players starts out with a momentum value of, say, 0. If a player accomplishes one thing (thing here meaning objective, whether it be to KO an opponent, or to get another pokemon in, or to get Spikes up, or whatever), and one thing only during a turn, then that player's momentum value remains the same. If the player manages to accomplish two things during a turn, the player's momentum value gains one point. If the player does not accomplish anything during a turn (wastes it), their momentum value goes down. Whichever player has the highest momentum value is currently threatening, and the player with the lower momentum value is responding.
Here, the threatening player is attempting to keep their momentum by accomplishing an objective every turn. The responding player is attempting to either gain momentum to try to catch up to the momentum of the threatener, or to cause the threatener to lose momentum. An example of catching up could be a Scarf Flygon outspeeding and U-Turning onto a weakened sweeper, thus KOing the sweeper AND getting another pokemon in. This gave the responder +1 Momentum Point (MP) because the responder KO'd a pokemon AND switched in another one. This Did not lose the threatener any MP, however. He did not KO any pokemon, but he did manage to switch in a new one for free, meaning he accomplished exactly one goal. Still, at the end of the turn, the responder gained +1 MP
An example of making the other player lose momentum could be sending in a Blissey on a purely special Starmie. The Starmie cannot KO the Blissey, which means that it is not accomplishing its objectives (Losing 1 MP) If it switches out, that wastes a turn, also costing it one MP.
Whichever player has more MP will win. Pokemon battles, I think are all about shifting MP from player to player. Here's an example of a a battle in which MP are used.
Battle Begins. For Brevity's sake, let's say each side only has 3 points.
A sends out Gyarados. B Sends out Infernape. (A's MP:0 B's MP:0)
Gyarados Uses DD, Ape switches out, Ampharos (I dunno) comes in (A's MP:0 B's MP:-1) <-- B wasted a turn
Gyara uses EQ, Ampharos is OHKO'd, B sends out Dragonite (A's MP:0 B's MP:-1) <--B's MP did not go down, because he did manage to get Dragonite in safely, but that's it.
Gyara Uses SE. It misses. Drag uses T-Punch. Gyara's Shuca Berry Kicks in, and it fails to KO. (A's MP:-1 B's MP:-2) <-- Both players wasted a turn. SE missed, T-Punch did not kill.
Gyara Uses SE. Drag Faints, B sends out Ape. Gyara OHKOs, and A wins.
/endfakebattle
I made this one really straightforwards, because I am lazy, but it's not a bad illustration of how momentum works. A had more MP throughout the battle, and so he won.
Note how momentum is not gained unless a KO occurs. Drag's T-punch did not give B momentum, but if it was an LO Gyara, and the residual damage eventually killed, then B would have gained momentum when Gyara was KO'd, not when damage was dealt.
Anyway, I think now would be a good time to list what constitutes an objective and what does not.
-KOing a pokemon
-Switching in safely
That's it.
But wait, isn't setting up an objective? Or dealing damage?
Nope. See, setting up actually is technically (by my system) not an objective, so you lose an MP. However, if you set up successfully, that means the opponent did not kill you (you did not get KOed, or he switched pokemon), meaning he wasted a turn as well. So it balances out, and no momentum is gained or lost. However, setting up does help your momentum by either ensuring that you do not lose MP (momentum) (after an SD w/ Lucario, you rarely fail to KO, meaning that you do not waste turns), or making your opponent lose momentum (after an Acid Armor, Vaporeon just will not die to physical attacks, making her harder to KO, thus wasting your opponent's turn.)
Same with dealing damage. If you deal damage, great, it makes you less likely to lose MP, because you'll kill next time.
Same with Status, Residual damage, Entry hazards, etc. They either reduce your opponent's momentum (Burn cuts attack, thus making them more likely not to KO, thus wasting turns), and Entry Hazards help you Kill, making it less likely that you lose momentum.
Basically, whoever has more Momentum (MP) by the end of the match wins. That's how I see it.
I'm not a very seasoned battler, so I'm not sure if this makes total sense. Please ask any questions you may have, or point out flaws.
So that's my two cents. I hope it wasn't incomprehensible or totally wrong.
What will happen is that I'll usually manage to get dual-screens up by good prediction, then memento my way out of there. Offensive pokemon are reduced to 1/4 of regular damage, allowing me to send out one of my slightly bulky set-up sweepers (Haxorus, Chandelure, Sciz), and get at least one CM or DD or SD up whether they switch out or (foolishly) decide to attack.
I think that this is a good illustration of momentum because in one turn, Uxie
-Allowed my Pokemon to come in safely
-Forced the opponent to waste a turn by almost negating its offensive stats.
It basically accomplished two objectives in a single turn, which, in conjunction, allowed me to become a threat. The opponent is now behind in terms of momentum, and is trying to end my sweep and playing defensively, rather than concentrating on trying to sweep.
If you'll bear with me for a second (this will sound a bit far-fetched), I believe that the concept of momentum shift in a battle can be summed up sort of like so. Let's say each of the players starts out with a momentum value of, say, 0. If a player accomplishes one thing (thing here meaning objective, whether it be to KO an opponent, or to get another pokemon in, or to get Spikes up, or whatever), and one thing only during a turn, then that player's momentum value remains the same. If the player manages to accomplish two things during a turn, the player's momentum value gains one point. If the player does not accomplish anything during a turn (wastes it), their momentum value goes down. Whichever player has the highest momentum value is currently threatening, and the player with the lower momentum value is responding.
Here, the threatening player is attempting to keep their momentum by accomplishing an objective every turn. The responding player is attempting to either gain momentum to try to catch up to the momentum of the threatener, or to cause the threatener to lose momentum. An example of catching up could be a Scarf Flygon outspeeding and U-Turning onto a weakened sweeper, thus KOing the sweeper AND getting another pokemon in. This gave the responder +1 Momentum Point (MP) because the responder KO'd a pokemon AND switched in another one. This Did not lose the threatener any MP, however. He did not KO any pokemon, but he did manage to switch in a new one for free, meaning he accomplished exactly one goal. Still, at the end of the turn, the responder gained +1 MP
An example of making the other player lose momentum could be sending in a Blissey on a purely special Starmie. The Starmie cannot KO the Blissey, which means that it is not accomplishing its objectives (Losing 1 MP) If it switches out, that wastes a turn, also costing it one MP.
Whichever player has more MP will win. Pokemon battles, I think are all about shifting MP from player to player. Here's an example of a a battle in which MP are used.
Battle Begins. For Brevity's sake, let's say each side only has 3 points.
A sends out Gyarados. B Sends out Infernape. (A's MP:0 B's MP:0)
Gyarados Uses DD, Ape switches out, Ampharos (I dunno) comes in (A's MP:0 B's MP:-1) <-- B wasted a turn
Gyara uses EQ, Ampharos is OHKO'd, B sends out Dragonite (A's MP:0 B's MP:-1) <--B's MP did not go down, because he did manage to get Dragonite in safely, but that's it.
Gyara Uses SE. It misses. Drag uses T-Punch. Gyara's Shuca Berry Kicks in, and it fails to KO. (A's MP:-1 B's MP:-2) <-- Both players wasted a turn. SE missed, T-Punch did not kill.
Gyara Uses SE. Drag Faints, B sends out Ape. Gyara OHKOs, and A wins.
/endfakebattle
I made this one really straightforwards, because I am lazy, but it's not a bad illustration of how momentum works. A had more MP throughout the battle, and so he won.
Note how momentum is not gained unless a KO occurs. Drag's T-punch did not give B momentum, but if it was an LO Gyara, and the residual damage eventually killed, then B would have gained momentum when Gyara was KO'd, not when damage was dealt.
Anyway, I think now would be a good time to list what constitutes an objective and what does not.
-KOing a pokemon
-Switching in safely
That's it.
But wait, isn't setting up an objective? Or dealing damage?
Nope. See, setting up actually is technically (by my system) not an objective, so you lose an MP. However, if you set up successfully, that means the opponent did not kill you (you did not get KOed, or he switched pokemon), meaning he wasted a turn as well. So it balances out, and no momentum is gained or lost. However, setting up does help your momentum by either ensuring that you do not lose MP (momentum) (after an SD w/ Lucario, you rarely fail to KO, meaning that you do not waste turns), or making your opponent lose momentum (after an Acid Armor, Vaporeon just will not die to physical attacks, making her harder to KO, thus wasting your opponent's turn.)
Same with dealing damage. If you deal damage, great, it makes you less likely to lose MP, because you'll kill next time.
Same with Status, Residual damage, Entry hazards, etc. They either reduce your opponent's momentum (Burn cuts attack, thus making them more likely not to KO, thus wasting turns), and Entry Hazards help you Kill, making it less likely that you lose momentum.
Basically, whoever has more Momentum (MP) by the end of the match wins. That's how I see it.
I'm not a very seasoned battler, so I'm not sure if this makes total sense. Please ask any questions you may have, or point out flaws.
So that's my two cents. I hope it wasn't incomprehensible or totally wrong.