It has a priority of -3. The user sets up a trap at the start of a turn, then deals damage if it is hit by a physical move, otherwise it fails. If Vivillon or another pokemon uses Powder on the user, it only activates after the effect is used. If the user is behind a Substitute or is hit by ally, the damage wont be enough to activate this move. Due to a glitch, if a Pokémon's use of Shell Trap is preempted by Encore to lock it into another move instead, and the Pokémon is then hit by a physical attack on any turn before Encore ends, the Pokémon's next use of Shell Trap will skip the step of setting a trap, and the move will go off successfully at its normal priority regardless of whether it was hit in the current turn.
|SMUSUM||The user sets a shell trap. If the user is hit by a physical move, the trap will explode and inflict damage on the opposing Pokémon.|
By Leveling Up
| Bold indicates this Pokémon receives STAB from this move.|
Italic indicates an evolved or alternate form of this Pokémon receives STAB from this move.