From what I understand, the watch can tell when it is touching your skin. The first time you put it on, you will need to enter a password. If someone grabs it off your wrist and it breaks contact with your skin it will require you to re enter your password.
That could work amazingly well, or be a complete and utter nightmare.
Many people don't like a watch tight. When I used to wear watches, I never had the strap tight, I always liked it a little lose so the watch could move a little, but not so lose it could spin around my wrist to the underside.
I wonder how tight/lose the watch will have to be to register a break of skin contact?