An employee relocated a stair stand with a tow tractor and was working to disconnect the tow tractor from the maintenance stand. The employee was manually operating the tractor's throttle to reverse and relieve pressure on the pintle hook. The tractor unexpectedly lurched forward, trapping the employee s left ring fingertip between the tractor s pintle hook latch and the maintenance stand s tow plate. The fingertip was partially amputated without loss of bone.