The way that I understand it is that it was common practice for, certainly Britishicon bombers, to commence their bombing runs together, stacked at varying heights, and bomb together once over target. The theory being that the bombs released from the aircraft above would most likely miss the aircraft below because it is a very big sky. Sadly the bombs didn't always miss the bomber flying below. One has to understand that there may be a thousand aircraft to get over the target, on a large raid, and one could very easily end up with total chaos with that many aircraft airborne at the same time.