I agree with BEagle, it is a response check.
Big Pistons Forever also correctly points out that the source of hot air is the exhaust and some form of shroud or scoop. Therefore increaqsing the heat of the exhaust slightly (or preventing it from cooling too much) will assist the operation of Carb Heat.
If it was more than a response check, one would be doing it on twins as they climb away simulating one engine failed!! or even during the whole arival, approach, missed approach and subsequent landing which is far longer than any PFL takes.
BEagle makes a very good point also - choose the area for doing PFLs carefully and allow for the posibility that the engine will stop at the worst possible moment. In other words if the student is making a really bad choice of landing area, stop it there.