Hel, that's very interesting. I've studied in India for a number of years practicing and later instructing yoga before I decided to go to college. That's the first time I've heard of feeling cold.
Relaxing and forcing blood by hanging upside down are two separate things. There's a lot of biochemistry involved with relaxation which I won't get into. But...as your blood vessels dilate, the oxygen affinity also increases with your blood cells. Other biochemical substances are released into your blood stream as well...and I haven't even gotten into discussing the brain activity during the alpha state...
But, like Brian, I am not closed to the possibility of other possible explanations. But if you're feeling it, then at least you know you're doing something right.
