[iMore.com]

Once you've unlocked your Apple Watch, wrist detection can keep it unlocked for as long as it maintains skin contact.

As security systems go, wrist detection is clever: It lets you have the convenience of accessing your Apple Watch without having to continually re-enter your passcode or password, but provides enough security to protect your data, including Apple Pay credentials under normal circumstances. In that way it's similar to Touch ID, which can unlock your iPhone or iPad based on your fingerprint.

Full story from the iMore Blog...