A Good Guy and a Bad Guy are fighting. The Good Guy is on the ropes and the camera cuts from a view of both characters to a shot of the Bad Guy from the Good Guy’s point of view just as the Bad Guy is about to finish the Good Guy off. Suddenly, the Bad Guy is shot/stabbed/decapitated/konked on the head from behind, and as the Bad Guy falls/explodes/turns to dust/etc. you see a second Good Guy standing behind, one who had been occupied with their own fight or apparently out of action a few shots back.
This trope has been used literally dozens of times on Supernatural–I wouldn’t be surprised if it was over 100–and just happened again last night. But it shows up in other action scenes, too.