Alan Greene is correct, the jaw should be pulled back. This relaxes the vocal apparatus for much more capable highs.
The jaw needs to be loose though, to produce the right emotional tone, and so, it’s difficult to just pull the jaw back regularly. It should be natural. This is accomplished be detensing the throat and balancing the head, such that the jaw hangs freely, without tension.
Singing and posture--see Categories, About, Launch 4-2014 