While you're shocking, it's not so much the higher active chlorine level that is the problem, but rather that you are shocking for a reason and you don't want people swimming if there's algae, excessive cloudiness (so you can't see them), high CC (which is irritating) or something using up chlorine even if you can't see it (since you don't know what it is and therefore don't know if it's harmful). So it's just simplest to say don't swim except below shock level (and with no more reason to shock) since that should only be happening after you are done with the shock process.