When your girl does a reach around and grabs your cock and with her other hand she uses two fingers in your ass. She simultaneously pulls back on your cock at the same time her fingers are inserted into your ass. Mimicking a shot gun.
A Baja shot gun is when you are in Mexico and you have one foot up on a rock and she does it. Enjoy...
A shotgun/baja shotgun is just like when you are mimicking a shot gun movement.