The jaw is a muscle that actually wants to drop automatically. And if one is relaxed, it would do so. This relaxation of the jaw should be used in singing. Let the jaw drop and pull it up. Most of the time, the jaw should not be pulled down strongly using the side of the throat muscles.
In singing, there are no pull-down muscles of the jaw. When opening the jaw, it is a pull backward. Gravity pulls down.
So, to sing well, must use gravity together with pulling backward, so as to maximize relaxation. This is the same as Alan Greene’s concept of a backward jaw.