Here is a diagram of the mods to the Heng Long recoil unit. I used a dremel tool with a rotaty mill to cut down the nubbins on the traveling block. Takes a couple of minutes.

Power is supplied to the motor at all times through the normally closed contact switch. When the bbl is in the extended position the contact is open cutting the power to the gun.

To fire the gun you hook the airsoft fire wire up to the motor. When you give the left stick upward motion, power is supplied to the motor. This moves the gear and frees the traveling block to snap back , allowing the contact switch to close, which then supplies power to the motor so the gun can continue its cycle. Once the gun is in its extended position it contacts the switch which cuts power to the motor, stopping the cycle with the bbl in the extended position.