When two people are licking each side of an erect penis shaft. They lick and suck their side of the shaft in an effort to get the guy off who is getting shaft battled. The person getting his/her dick sucked is the one getting his/her shaft battled over, and he/she decides the winner, and who subsequntly gets the load. One may challenge another to a shaft battle, as long as they have a third party shaft to battle over.
Joey challenged Shane to a shaft battle, so together they shaft batled Jordan. Joey won and got the load.