It seems clear that using a regular hand posture is not possible. First of all, if you want to spin on TOP of the tip of the thumb, you have to spin palm-up.

Secondly, depending where you place your thumb, the pen can hit the palm or the other fingers. This problem could be solved by using shorter pens, but this is not a practical solution.

If your hand is big enough, one good posture would be to form a "V-shaped" with the index and the thumb:



Another way is to place your hand flat and the fingers all near each other:



Finally, we could make a fist and stick out the thumb:



All those postures try to alleviate the problem of the pen hitting other fingers or the palm.


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