The way I was trained, many years ago now, the punch-back is designed to indicate to the opposite wing if the player on the far side is on-off the line to allow each wing official to get a correct count of men in the backfield. The one (or two) that are tough to determine from the opposite side of the line are the widest players. If one or more inside players are back with the split end up on the line the opposite wing can see that since they are "behind" the further out split end so we have no signal. Given that, we've adopted a mechanic that if the end(s) is/are not on the line, we give a punch back fist for one in the backfield, and a two finger punch back for two "ends" in the backfield. If after the punch back is given the widest resets up on the line we give a fist to the chest signal.
Our supervisor also considers it a "miss" if we have a formation foul for more than 4 in the backfield but we don't have flags down on both wings.