[hteas]

Sean,
You state that contractions are nearly impossible with negatives. Is that an assumption or do you have any information you could share?
By the end of a 120 second negative (my best) I have been assuming that the concentration of CO2 in my blood is pretty high, but may be wrong. if you are right then the training is all based on hypoxia. I've been assuming that it was a hybrid with some aspects of both hypercapnia and hypoxia, since I limit breathing to 60 seconds between holds.
Howard