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 This effect resets when the holder is withdrawn.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.
If a Pokémon holding a Choice Scarf Dynamaxes, the item will temporarily have no effect; the stat boost will be disabled, and the Pokémon will no longer be subject to move locks. After the Dynamax ends, the stat boost is restored, and any previous move lock will be re-enabled.