If you are talking about the SPZ 7600 AFCS, I thought that this button was originally marked 'HOVER' and on later versions of the system became renamed as 'VELOCITY'. It's a nice way to allow engineers to fly the A/C smoothly at low speeds, beeping it into a hover with the trim button. I've not seen it used by real pilots too often though, I guess they can probably hover the aircraft using their own on-board accellerometers!