The main rotor blades in nearly every small helicopter are belt driven and there is no direct coupling to the engine and the blades do not act as a fly wheel would or as in fact a propeller that is directly coupled does.
I think one of the air safety magazines caused a bit of a stir last year when they suggested you could check which mag was not working whilst flying by switching to right and then left. If you do this in a small piston engines helicopter with one mag working there is a very good chance the engine will just cut out. In a plane that would be unlikely as the propeller would keep it turning. Feel free to try it out.