The ability to masturbate skillfully with both hands. When one hand is tired, the other hand may be used to continue just as effectively. It is also possible to create unique combinations using both hands.
Guy 1 - I was trying to fap last night, but my right hand got tired, so I had to stop.
Guy 2 - I don't have that problem; I'm fapidextrous. I can go as long as I want.