when externally applied heat shocked open the radiators in the cold palms of anesthesia patients, warmed blood was returned straight to the heart, and the body was reheated from the inside out. Applying a mild vacuum to the hand intensified this effect.
It doesn't need a vacuum, but it does help. They don't seem to mention why.
Applying a vacuum to an area causes blood to well in that area, suck on your hand and you'll see the area you suck on blush. So presumably you're making capillaries open up and increase heat transference over the skin threshold. Opening all the capillaries in an area will increase the volume of blood in that area too, so greater volume and greater heat gradient (over a small distance).
Disclaimer: Assumptions, not medically/biologically trained.
