The user of Petal Dance attacks for 2-3 turns, during which it cannot switch out, and then becomes confused. Confused Pokémon have a 50% chance of hurting themselves each turn, for 1-4 turns. The damage received is as if the Pokémon attacks itself with a typeless 40 base power Physical attack.
If Petal Dance is disrupted (e.g. if the move misses or the user cannot attack due to paralysis) then it will stop and not cause confusion.
Pokémon with the ability Own Tempo or those behind a Substitute cannot be confused.