When held by a Pokémon, it boosts the Pokémon's Special Attack 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 This effect resets when the holder is withdrawn.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 Band 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 Band, if the held Choice Band is removed while its effect is negated, the Pokémon will be affected by the Choice item lock glitch.
If a Pokémon holding a Choice Band 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.