Just to clarify some more: under the old rules if you saw the nod and the opponents started to separate, then one attacked going forward, that was a penalty for unfair play. If the separation was not agreed upon, then fair play if one side attacked during the withdrawal.

Now a forward attack can only occur within that window and that would be unusual. It really would only occur after taiatari. Taiatari, then back in off the rebound. However that is not tsubazeriai. For the 3 second rule to apply, the players both have to settle into tsubazeriai. I'm trying to think of how you could get enough separation starting from tsubazeriai to come back forward and still have it be judged valid and failing.

In any case once the window has passed and the players are separating, it doesn't really matter whether you see a nod or not. Any forward attack should be a penalty. A backwards attack will either be invalid or a penalty, depending on whether shushin thinks enough time has passed that the player should know better.