Luke, I think I understand your confusion. You're right, there's a limited range of ankle tipping available. So you're confused about what keeps you developing angles after the ankle of your inside foot is up against the lateral-side wall of the boot, which will happen as soon as you start to tip. The answer, if I may be so bold, is that it's the continued pressing of that ankle against the side of the boot (or, if you prefer, the continued pressing of the side edge into the snow) that keeps the kinetic chain activated, and thus causes the hip to progressively move down towards the snow. [As geoffda explained to me in another thread, "While the hip must move laterally inside the turn, the primary direction of movement is down."] I.e., the ankle doesn't have to move more,it just has to keep pressing in the same direction. The analogy I'd use is the gas pedal of a car. Once you press it to the floor it can't go any further. Yet the car will continue to accelerate.
It's because of the need to access the side of the boot that PMTS requires footbeds that have some flexibility, i.e., that don't lock the feet. For more information, see:
viewtopic.php?f=1&t=1660&p=15818&hilit=eversion+tipping+inversion#p15818