Thrust reversers. They are used all the time upon landing, to slow the aircraft and save the brakes. Never have seen it used for backing an aircraft, but somewhere on the globe they probably come in handy occasionally, in lieu of a tractor to push the plane around.
Once at Oshkosh, I saw a C-130 (Fat Albert) change the prop pitch to back the plane out of it’s parking spot.
Many years ago, when Eastern Airlines was still a going concern but was facing financial difficulties, they started making their pilots use the thrust reversers to back out away from the jetway, rather than having the ground crew use motorized equipment to handle the pushback. The pilots were not amused. Neither were the passengers.