I've made similar inquiries at www.holdyourbladder.info, and my overview titled, "Bladder Capacity in the Practice of Water Holding" is available there.
Generally, the bladder's detrussor muscle goes through a process of overall stretching by contracting in spots, which are felt as surmountable urges, until a fully coordinated contraction overwhelms the sensing part of the organ, the bladder neck (B). That's the scenario everyone here is probably most in tune with, and you can have faith in its inexorability.
But many girls do have to release intentionally at high volumes, because there are various ways that the sphincters can gain a mechanical advantage (A), most commonly via collapse of the bladder neck, which is like folding the outlet shut instead of constricting it with the pelvic floor muscles. It's not necessarily just a matter of "letting go" at that point—she'll probably have to push with her abdominal muscles.