How about a tiny tube hooked to an aquarium air pump blowing air on the bottom of the rear rotator causing it to spin.
So looking at the OP responses on the original thread, the main rotor spins because he is blowing on it.
*Edit quote here.
"Hah no, it's just spinning because I was blowing on it. Usually they'll have you glue it down, but the way the kit made it allows you to glue just the cap on if you're careful."