Well, yes, I understand why he does it, but I don't think it achieves the end people think it does. Someone is most likely to let something slip when they are relaxed and at ease. When they're primed, they'll be much more careful and circumspect and if they do make mistakes those mistakes will mostly just be mistakes due to poor thinking rather than admissions of anything.
Like when you keep barking at someone who's driving, sooner or later they will drive badly and risk having an accident. This doesn't prove that they are bad drivers, just that your behaviour oppressed them to the point that it intefered with their ability to drive (read answer correctly) and they could no longer effectively drive.
It's the same tactic used to extract false confessions, so go figure.