A piano whippen is the main working mechanism in the piano action that incorporates the jack which is the escapement mechanism allowing the piano player dynamic range. When you press down on the key, the whippen is responsible for transferring upward movement bringing the shank and hammer to the string. The jack is contained in the whippen and the jack is the escapement mechanism that allows the shank and the hammer to travel the last 1/16 of an inch on it’s own inertia, thereby enabling the player to achieve pianissimo or forte tones with a soft or a hard hit.
The whippens below are from a Steinway grand action.