Say if by gripping a pen, the thumb is placed on top of other fingers (the index finger in the example below), instead of touching the pen itself, how would you name that position? A short expression with as few words as possible is appreciated.
This is called a thumb wrap according to these experts.
It looks like:
someone holding a pen in his fist.
It's not entirely accurate, and also implies a tight grip, but does convey that it looks awkward.