"It all comes down to sensors. The sensors on the Apple Watch will know if a device is being worn on a wrist and when it is taken off. If you take the Apple Watch off, it will lock itself and can't be used for payments unless a code is entered into the watch. Additionally, taking the watch off and putting it on again will mean you need to enter in a security code.The Apple Watch Knows When It's On or Off Your Wrist
We're sure Apple will figure out a way to make this entry process as seamless as possible — as to not be annoying — but it's good to know the watch can tell when it is on a wrist or just sitting on a desk or charging pod."
Friday, September 12, 2014
The Apple Watch Knows When It's On or Off Your Wrist [Mashable]
See the full post for more Apple Watch feature details