Forgive me for being late on entering this discussion, but I feel there is a relevant point to be made.
The OP asked about how and if overbreathing a reg. could happen. The replies have explained - to some extent - the answer to his question. Rather than get all technical about regs and the mechanics of breathing I respectfully suggest that, if such a perceived problem were to occur, then pressing your reg purge button SLIGHTLY will accomplish two things:
(a) Overcome the breathing resistance needed to "crack" the valve's diaphragm each time you breathe;
(b) Supply an immediate psychological placebo in that you will realise all the air one needs is immediately available, hence overcoming the "escape to the surface" syndrome most people get with such an experience.
It works every time, allowing the "Stop, Think, Act" philosophy to be carried out with less anxiety - and more air!
If you want to test this theory, then "sprint" awhile when next you're on a dive. Then, as you begin to pant a bit, try the above.
Have been doing it for years where necessary, and always inform any students.
It can be a lifesaver.
Cheers,
Seadeuce