The stump jumper is a plate or disk that is usually round although I believe there are a few elliptical plates on some models. The swinging blades are attached to the stump jumper. The swinging blades will hopefully pivot when striking and immovable object when mowing. The stump jumper will in theory travel/"bounce" over the obstacle without damaging the mower's gearbox output shaft. It can be quite noisey when a RC engages an immovable object. Jay