Pokémon Black Version and Pokémon White Version - Priority and Turn Order
Turn order is determined two times each turn: once after commands are chosen, and again at the end of each turn (before end-of-turn effects).
In Rotation Battles, if a side chooses to rotate another Pokémon in, the turn order is determined according to the Pokémon that will be rotated in.
Because of a bug, however, Trick Room’s effect treats a Pokémon with a current Speed of 1809 or greater as having a Speed of 0, thus allowing such a Pokémon to strike before those with Speeds higher than 0 while Trick Room is in effect.
After commands are chosen, a Pokémon’s Quick Claw can be activated at a 20% chance, but only if it chose a move or the Shift command (without rotating out or being rotated in) that turn, or didn’t take any action this turn. If it activates, the Pokémon strikes before all other Pokémon with the same or lower priority level. This effect doesn’t apply to the end of the turn. (Each Pokémon with a Quick Claw has a separate chance to activate.)
After commands are chosen, a Pokémon’s Custap Berry is activated (and is consumed) if it has the required amount of HP remaining, but only if it chose a move or the Shift command (without rotating out or being rotated in) that turn, or didn’t take any action this turn. If it activates, the Pokémon strikes before all other Pokémon with the same or lower priority level. This effect doesn’t apply to the end of the turn.