I've been on a number of retreats where the primary focus was to cultivate the brahma viharas.
I went through a phase of a year or two when, at retreats (although not in my daily practice) I would consistently develop a really hot head during sits and walks. My cheeks would become very flushed (confirmable by a mirror) and hot to the touch. It felt like heat was coming off my head/all my focus was going up through the crown of my head.
At the time, I was concentrating on concentration, as it were, and wasn't really interested in this--I simply observed that this phenomenon would occur, but would never stop and persistently investigate its Three Characteristics.
It went away, but I am curious--what was it? Because it's gone I can no longer investigate it. I am thinking perhaps it was an artifact of 3 Characteristics. Maybe it was my version of arm-flapping.
In terms of getting that sort of thing back, energy phenomena come and go, but tumo breathing practices and the like can make them more likely.
Thank you. I'm not sure that I want it back although I suppose it would be handy if I were stuck in a snowstorm. I occasionally see traces of the phenomenon when I am concentrating hard at work, but usually take it there as a sign to back off and have a glass of water.