A flat spot is filed or ground on the side, the length and diameter of the pin. ( I used a transfer punch and a finishing nail with the head cut off for my first pin chuck) When it's inserted into the tube the pin goes to one side or the other and jams in place.
A stripped 4mm truss rod nut was found. A one inch piece of .017 guitar string was inserted along one of the flat sides of a hex wrench. It jammed just like a pin chuck allowing the wrench to both tighten and loosen.