This is the "stack and track" part of the NPA's mechanics model. Head and shoulders remain stacked upright through shoulder rotation and external rotation of the throwing arm. The low back extends (ie. arches) in a momentary isometric load. About the time the arm snaps forward, the low back releases and the torso flexes forward. While all of this is happening, the torso continues to track forward.
Now, from what I gather, too much of this and the rotational (i.e. Nyman) guys would say you're too "linear" and that's no good. This is at least part of what they claim caused Prior's injuries (as opposed to the infamous "inverted W". Yet another unsubstantiated claim, IMHO.
I believe good momentum certainly contributes to what you're asking about. But there are other things that contribute as well, such as good posture and balance, good timing, good hip and shoulder separation, etc.
I don't think you should focus on getting your head out over the front leg. Just like I don't think you should focus on getting your foot out front to increqse your stride. Instead, you should focus on the things that let the head get out there as a result.
The question is why don't you do it? Is there a problem preventing you from doing it or is it just natural?