The secret part of the prop or the secret object that makes the trick work.

Example: the extra cap for the Ball Vase

GAFF

A gimmick designed to look like something ordinary,

while in reality it is accomplishing some secret task.

Example: the Bandit Ball box, made to look ordinary

but of course it's not

SELF WORKING TRICK

A trick that requires minimal skill and no sleight of hand,

almost like it works by iteself.

Example: the In a Flash Box where you just

lift off the box cover and the magic happens by itself