To get a recoil to function requires three wires.

Your poles on the motor are wired to the RX 18 through the smoker jack, These have to be wired to give you the direction you need out of the motor. Once you have the direction right you consider the wire going to the contact swtich, "Negative"
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Now you have to figure out what triggers the gun. In my case I use upward movement on the left stick(HL). This is the wire that feeds negative power to the airsoft motor. Instead I use it to trigger the recoil mechanism. You can wire this diretly to the negative terminal on the motor, or some contact switches have a post you can wire it to.

Thats all there is to it.