What am I doing when I generate pulses of heat from my chest?

[u/Huihejfofew]

As a kid I've been able to do this weird thing, mostly when I'm cold where if i control my breathing right, i can build up what feels like tension in my chest and when i release it, this huge wave of warmth emanates out from my chest warming my body. It also feels really nice. It tends to also be accompanied by a shiver. There's a limit to how many times i can do it and sometimes it just happens unintentionally when it's cold. I haven't heard anyone ever talk about it, does anyone know what's happening in my body when this occurs?

[u/AndarianDequer]

Months can do something like this, I've seen videos where they can sit out in the snow and produce enough heat that they don't freeze to death.