I assume that it is a judgement call about when forward progress is stopped, and the whistle is blown, and the tacklers should disengage? I saw a play today like this and the players were, maybe 5-6 total from both teams, all going backwards when the group collectively fell to the ground. I was on the sideline close to the play, heard the whistle, and saw the players fall. It was bang, bang, not drawn out. But, the flag flew. What would the official need to see to call this?