I found this in a search. Don't know if it is true, just throwing it out.
"It is a common technique used in martial arts training, to point your index finger up to the wrist of the person you are shaking hands with. it does not affect the solidity of the grip, but what it does is inhibit the other person from trying to squeeze or crush your hand should they wish to get aggressive. also it places your finger in the perfect place to counter with a pressure point strike, which can bring the person to their knees."
"Basically it's a defensive posture, which after learning you just get into the habit of doing normally and without even noticing it."
http://www.abovetopsecret.com/forum/thread576908/pg1