When in a three way and cumming on the girl (either boobs or face) the area in which cum might shoot is considered the blast zone. The other guy is to stand back and whack while until the other guy finishes so he doesn't get hit. When the man is finished the other person approaches and the just finished person backs up to respect the mans Blast Zone.
Man 1: Dude, Get outta my blast zone!
Man 2: Sorry man!