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After attacking the opponent, the player chooses a new Pokémon to send out. If this move KOs the target, the user decides which Pokémon to send out, then the opponent decides. If there are no more Pokémon in the user's party, this move only does damage.
In Diamond / Pearl, if the Pokémon switched to has a Choice Band, Choice Scarf, or Choice Specs, and also knows U-turn, it will be forced to use U-turn. In Platinum, the Pokémon switched to is free to use any move, even if they are holding a Choice Band, Choice Scarf, or Choice Specs.
Pursuit hits a faster Pokémon using U-turn for double damage, but has normal power if the Pokémon using U-turn is slower. Against a slower Pursuit Pokemon, the U-turn Pokemon damages the Pursuit user first, then the Pursuit user damages the U-turner, then the U-turner selects their new Pokemon. This means that if U-turn faints the Pokemon using Pursuit, the U-turner does not take Pursuit damage.
If a Pokemon under the effects of Ingrain uses U-turn, that Pokemon is allowed to switch out, and the Pokemon that replaces it does not gain the effects of the Ingrain.