like the creampie, only some one ejaculates into the anus. The receiver then squeezes as if they were about to take a massive dump, ultimately shooting cum out their ass like a rocketship exiting atmosphere.
It's when one has their significant other drink/eat laxatives, and perform anal sex until you feel the pressure, then when you do pull out fast and move. That is how you do the Montana Mud Rocket
"Dude Sally and I performed the Montana Mud Rocket"
"Bro that's gross"