In reply to ClimberEd:
restricting blood flow thru the femoral artery maybe?
if youre legs are a bit pumped beforehand, therefore stacked with blood, it may be reducing your blood pressure as it cant get back to the heart fast enough...?
i recall a technique in thai massage where they hold your leg up over your chest to do something like this. for what, i dont know exactly.
i could be completely wrong.