Why did the BBC's Director General keep nodding in agreement with himself?

Watching George Entwistle giving evidence to the House of Commons select committee on culture media and sport reminded me of something my late sister-in-law used to do, the meaning of which we never managed to work out.

Sometimes, she would mark the end of what she'd just said with a rather emphatic "Hmmm!". The nearest we got was that it meant something like "that's it and I don't want to be asked anything else about it" - because we all took it as indicating something final about her comment on which she'd rather not have any more discussion thank you very much.

My followers on Twitter may have noticed that I became rather preoccupied (and seriously distracted) this morning with Mr Entwistle's obsession with nodding in agreement with what he'd just said (e.g. 28 and 1:06 seconds in the above). It wasn't just that he did it occasionally, but did it after almost every 'answer' to every question.

It may have been his way of telling the committee that he'd no more to say on that particular matter, but suggestions and/or enlightenment from readers would be very welcome.

This sequence also includes the only question of the morning that prompted raucous laughter from those in the room (about 1:10 seconds in). It prompted an embarrassed-looking grin from the Director General, and was enough to prompt one contributor to Twitter to describe it as 'humiliating'.