On the original B&S universal head, the worm is disengaged by being held in an eccentric carrier that when rotated pulled the worm out of mesh in a radial direction; on early models this was done with a knob that when rotated turned a pinion gear that engaged gear teeth on the eccentric carrier, when reengaged, there was a locking knob that prevented the pinion shaft from rotating, later units (the one that I have has a lever directly attached to the eccentric carrier and has a hinged arm that secures the knob in place.