For instance, if you were playing Call of Duty splitscreen with three other people on Team Deathmatch, and you were going into the enemy team's base, and you wanted cover, you would turn to your teammate and say, "Hold my ass". He would proceed to give you supporting fire.
Jake: "I'm going in through the rear. Hold my ass, and hold it tight."
Callum: "Don't worry; your ass is safely in my hands."
Ben: "I'm gonna go pwn Jake, he's entering from the rear."
Andy: "Look out, Callum's holding his ass!!!!"
Ben: "HOLYFUCKINGSHIT!!!!! *gets pwned*"