Focus Punch is a decreased priority move. The user of Focus Punch will start focusing at the beginning of the turn the move is used, then execute the move Focus Punch at a priority of -3, unless it was hit by another Pokémon's damaging move before executing Focus Punch, in which case it will lose its focus and be unable to attack that turn. The user's focus will not be broken if its substitute is attacked or if it is hit by a status move.
Its charging message is displayed before any other moves.
PP is consumed even if the user loses its focus and fails to execute the move. Even if the user loses focus, it is considered the last move used (for the purposes of Encore, etc.).
If a Pokémon becomes locked into Focus Punch by Encore but selected a different move that turn and hasn't already moved, it will use Focus Punch at the priority of the move it selected, but the charging message does not appear and it will still lose its focus if it took damage from an attack that turn.
If the user is hit before the attack, it will say, "<Pokémon> used Focus Punch!" before it says the "<Pokémon> lost its focus and couldn't move!"
Generation V onward
It no longer states "<Pokémon> used Focus Punch!" preceding "<Pokémon> lost its focus and couldn't move!"
PP is no longer consumed if the user loses its focus and fails to execute the move. If the user loses focus, it is ignored when determining the last move used (for the purposes of Encore, etc.).
If a Pokémon becomes locked into Focus Punch by Encore but selected a different move that turn and hasn't already moved, it will use Focus Punch at the priority of the move it selected regardless of whether it took damage that turn, and the charging message will not appear.
The user focuses its mind before launching a punch. It will fail if the user is hit before it is used.* The user focuses its mind before launching this attack. It will fail if the user is hit before it is used.*
Focus Punch has a charge-up turn (akin to Skull Bash, Sky Attack, etc.), and can hit even when the user is hit during the turn of charge-up, unless the user is inflicted with a Status condition like paralysis or sleep.
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Since Focus Punch has a higher priority than Avalanche, Revenge, Counter, Dragon Tail, or Circle Throw, they are unable to break the focus of a Pokémon using Focus Punch. Thus, they and Focus Punch itself are the only attacks that can strike a Pokémon in the same turn it successfully uses Focus Punch, without the use of Quash or After You.
In Generation III, a glitch will cause the user of Focus Punch to prepare to use it, but not execute the attack. This glitch is triggered by selecting Focus Punch in a Double Battle on the first Pokémon, pressing the "B" button, then switching out the first Pokémon. When the battle is conducted, the first Pokémon will tighten its focus in preparation for Focus Punch, then switch out. This is due to charging turn having a priority of +8, and therefore being executed before switch out.
Shroomish and Togekiss are able to learn Focus Punch, despite lacking any visible hands to use the move. In Shroomish's case, it is likely explained by the fact that it evolves into Breloom, which not only has arms, but also is a Fighting type. The fact that Togekiss can learn the move is likely due to the fact that it evolves from Togetic, a Pokémon that does have hands.