It is somewhat helpful to think of your body a bit like a turtle shell where what would be the shell side (the line from back of hand to shoulder) is _not as bad_ for getting cut as what would be the underside (the line from palm to arm pit).
Regardless, the thing about knives is that once cut, the other guy typically just has to play keep away for under 2 minutes while you bleed and continue to weaken.
If you are going to take a knife when you are unarmed, the only realistic chance is to develop center on contact, and once you have that (how many have that?!) you need to make sure to grab the meat of the thumb as opposed to the wrist before attempting stripping the knife.
But that is still very stupid. Roll up a magazine, use a belt, a pen, a comb, a flashlight, throw your keys at them; do SOMETHING other than try to take that knife barehanded.
However, I say just invent wallets that explode when more than 5 feet from you and be done with it.