When a male ejaculates, and his sperm shoots out and land in his hole (A.k.a. bellbutton). When this occurs, one would say "Fire in the Hole!"
This term is also popularly used in the context of war. The phrase "Fire in the hole" would also be correctly used when throwing any object to a certain destination point, but mainly the reference is to throwing a grenade into enemy territory.
Male: As my girlfriend (or boyfriend) was giving me a hand job, I blew my load, which shot off into my bellybutton. To this I shouted, "Fire in the Hole."
Sergeant Johnson: Fire in the Hole! (Then proceeds to chuck grenade away from foxhole and over to enemy territory).