Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

You need an overrun mechanism, the drill engages to turn the engine and once it starts the coupling is automatically disconnected, think vintage car hand cranks.

I agree that you could trim the drive side to make it release as the engine fires, but Jin is just pulling the drive side off in his video.  I'm not fond of having the ball side of the u-joint attached to the crankshaft as Jin does, which is why I went with a pulley and string method to start mine more easily.   Another solution would be to use something like a model airplane starter - a friction cup on a spinner.  I'm going to hang with my low tech string - I'll always have one available if I store a spare string in the base with the batteries.