When held by a Pokémon, it boosts the Pokémon's Speed by 50%, but only allows the use of the first move selected. This effect resets when the holder is withdrawn.
In Pokémon Diamond and Pearl, due to a glitch, if a Pokémon uses U-turn and switches into a Pokémon holding a Choice Scarf that also knows U-turn, that Pokémon will be locked into U-turn. This was fixed in subsequent games.
Generation V to VI
From Generation V onward, if a Pokémon is already locked into a move due to a held Choice Scarf, if the held Choice Scarf is removed while its effect is negated, the Pokémon will be affected by the Choice item lock glitch.
If a Pokémon holding a Choice Scarf uses a move other than the one it's locked into (such as through Instruct or Dancer), the move will fail and no PP will be deducted.
||An item to be held by a Pokémon. This scarf boosts Speed, but allows the use of only one kind of move.
||An item to be held by a Pokémon. This scarf boosts Speed, but allows the use of only one of its moves.
|An item to be held by a Pokémon. This curious scarf boosts Speed but only allows the use of one move.