This is a scenario where a regressed fur still has undies that still fit them. Whilst they are confused, someone sneaks up behind them, pulls the back of their briefs out, and snips the waist band with a pair of scissors. This prevents them from holding onto the host's bum and makes it easier to diaper them since you don't have to struggle to rip their undies off. I kinda got this idea from one of the All Dogs go to heaven movies. In one song sequence, the antagonist was reduced to his briefs and one of the devil's minions snipped it off with a pair of scissors. Don Bluth is a master of animation by the way.