When dad realizes his teen sons are horny all the time and jerk off alot like he did as a boy, he gets them a hooker and rents a motel room. Dad can then watch his sons fuck and get sucked off by the girl or join in with his boys and blow some massive loads of sperm.
Boy #1: Dude, dad got us a hooker again for your birthday this time! Boy #2: Sweet! It's bro bustin time! Dude I got a compliment from the girl last time how good my sperm load tastes. LOL
Bro ball bustin means sharing a girl with your actual brother or friends close enough to be considered bros. A mmf situation, the guys fuck the girltogether, dp, spit roast or one dude fucks her while the other gets head.
One bro comes home and says to his brother, "Dude get ready. A girl is coming over while mom and dad are out, " he says while starting to take off his shirt and strip down. "Get the condoms, it's time for some bb bustin!