The word that immediately came to my mind when I read this scenario was 'calibration'.
Overhearing Corbin's remarks, and seeing his general demeanour, provided some useful feedback on how how effective the spanking I gave him was. It has obviously had less of an effect than I had intended though he is lying on the bed on his front which suggests it certainly had some effect. Next time I will have to spank him harder and/or with his briefs down.
Do I spank him again, now? No, that I didn't get the level of severity right is my problem not his. Do I let on that I have overheard him? Probably not, his loose tongue is a useful source of calibration feedback. At the first suitable juncture I tell Corbin that if I have to spank him again his underpants will have to come down. It is the most unambiguous way to indicate an escalation in the level of punishment, especially if big brother Stephen is normally spanked this way.
Of course there is the possibility that my original spanking has more of result than I realised and Corbin's words were just bravado to impress his brother in which case I risk overcompensating. So when I tell that he is moving up the bare bottom spanking I will have to pay careful to his attention to the news as well as his reaction to that next spanking to make sure I don't overdo it.