https://static01.nyt.com/images/2016/03/24/business/24artifi...
As a completely uninformed guess, I think the link between focus and smelling one's hand/wrist might have something to do with early feelings of comfort as an infant associated with the smell of a parent's skin. The idea being that introducing a close approximation of that smell would remove any distraction brought on by other, less familiar smells, and increase focus.