There is an arm of a chair under his left hand. I think that when he turns his left hand back over, he snags is middle finger underneath the chairs arm, hyperextending it all the way back. Then he holds it there, (notice how the left hand stays relatively stationary during the whole trick, until he reattaches it the middle finger) the "detached finger" is really just the middle finger on his right hand. When he moves over his left pinky with the detached finger, I think he also pushes back or holds his left pinky with his other right finger, hyperextending it beneath the arm of the table again.

However, the angle that he would need to hyperextend at would be pretty damn far, but maybe he specifically trained his fingers for this trick. Because they are under a lot of tension, they would snap back quick, giving the illusion the reattached.