She does. I do too. I find it impossible to stop even when people sit on my leg to stop me. As habits go it's not the end of the world. I'd flick my foot through Lens Lens if I had to sit through it too!
And to be fair to Darcey, she did explain why she did all those yahs to begin with. When she's coaching dancers she is always looking for a response, to be sure they've got it, hence the question after every sentence.
Personally I think it's time to leave her alone and let her get into the role; I can't imagine anyone - even the people mooted as replacements - would do a brilliant job right off the bat - tv just isn't like that.