To have one's back is to protect one from unanticipated matters, while one is attending to a particular function. This can be seen dramaticallyand concretely in the movie, "Gladiator." THE ACTOR forges an alliance when he suggests that three gadiators form a team by fighting off threats from different directions (fighting back to back.) He states: "We can fight together or die separately." Also, in the revolutionary war period, one of the notables said "We can hang together or hang separately."
Similar idioms would be HAVE MY BACK= "cover me," "fight back to back," "back me up," "play chicky for me," "keep an eye out for me," and "be ready to jump in."