bb56 is right about the motor having a hollow shaft where the servo to change the prop from positive to negative angle sat behind the motor and the wire went thru the motor hooked to the blades. No reversing the esc. Use graphite to reduce resistance of motor.
You set the radio up in heli mode so RPM"S are fixed and center throttle is 0 degrees prop. If you advance the throttle, prop goes positive and if throttle is less than half, prop reverses. If you fly helis and can hover inverted, it is the same thing.
Why 4D is not popular is that the stresses the prop and rig take were made out of plastic!!!! I went thru 3 different companies and each blew apart from stress. Prop design is critical(to have positive thrust both ways) and cheap plastic doesn't do it. Bet those guys change parts often. Now made from metal and adds terrific weight to model. That hinders it.
For those who have RealFlight6(others ?) look for FlatOut Extra with V (variable) Pitch. Have a go at it. Gives you the chance to see how good you are.
I like to design and fly unique planes.