Similar to masterbating, but once one busts, they will put in all of the weight and pressure in the body into the penis, and even stroke it with a fist or other objects at the same time as-well. This will cause the penis to make as big of an explosion as possible and cause the ejaculate to splatter and fly all over the place, similar to a bomb.
"Finished Masterbombing in my room yesterday. Took me almost an hour to clean it all up, and my weiner still feels weird."