Hi Ernst,
Odd question here - What is the difference between Gemini who lives in their head oblivious to the physical body, and a yogi who exposes themselves to harsh environments oblivious of their bodies?
I don't think gemini lives in their heads. Its afflicted moon that most often lives in the head or an afflicted 4th and 4th from moon. Any bad lajjitaadi avastha will cause a person to go to their heads instead of their bodies.
The difference is that when someone lives in their heads to try to escape their wounds by managing with their minds nothing works from doing that. when a yogi puts their body under duress its to shift the bodies energy inward into the chakras and up the spine into the brain in order to have a spiritual experiences.
Sorry, with being in their heads, I meant mental activity, trying to figure out solutions to problems. But it is possible that I have understood it incorrectly or remembered only parts of what you taught, and need to go back to basics.
Is the 4th always count forward from moon, regardless even or odd rasi?
Aren't Gemini's good at things like tennis?
no, that is too simplistic.
to be good at tennis you need grace of movement which is venus and some power bursts which is mars. Then as its a game you do need mercury related to these planets somehow such as mars in a sign of merury and the third house for competitions. See john McEnroe who has exalted venus as lagna lord and mars and moon in gemini rasi aspecting with lagna lord and 3rd lord of competition jupiter.