While performing cunnilingus, the vagina is blown into, loading it up with a pocket of air. The performing party then grabs the woman's ankles and separates them in a manner resembling holding the handle bars of a low-rider motorcycle. The resulting queefs resemble the sound of a chopper.
Additionally, the ankles can be twisted for that extra bit of immersion.
Dom's mom loves the west coast chopper. Especially from Nick.