IMHO this is not metaphysical in any way.  My girlfriend used to feel very cold a lot of the time.  Then one time I took her to walk barefoot on a glacier in Iceland and she really liked it, and not feeling cold anymore for the rest of the vacation (Iceland is not that cold anyway, just very windy.)

Eventually, back home, she did one of the Wim Hof courses, including bathing in a bath filled with ice cubes and she experienced the "inner fire".  AFAICS it is all about relaxing and letting the body do its natural thing, letting the body's thermostat click into action.

To think that this is exceptional is IMHO mostly a testament to the extent that we have lost contact with our physical nature and origins.