Start your nitro R/C engine effortlessly using this new Roto Start with powerful 550 motor. Insert the 6mm shaft with hexball tip into the new Torque-Slip design backplate receptacle to start your engine. The torque slip back platefeatures a specially design 'slip' unit to avoid possible damage should the engine lock up. Two different back plates are available for HoBao Hyper 21 and SH/Force engines.