A component of the Wurlitzer piano action. Its role is to hold the hammer which strikes the reed by being pushed by the lower action (called the whip assembly). It works by the key being pressed which triggers the whip assembly which in turn pushes the fly towards the reed. On the underside of the fly is a recess that catches the jack of the whip assembly after the fly hits the hammer. This allows the fly to lock into position after the key has been pressed so that the reed is not struck again. Its effect is to allow maximum control when pressing the key.
Its design is inspired by a grand piano action, where a jack pushes the piano hammer upwards to strike the string. The main difference, however, is that a grand piano action has a knuckle that acts as the catch, and no recess on the underside of the hammer shank like the Wurlitzer fly.