One of the most anticipated features of macOS Sierra may actually be one of the simplest: Auto Unlock, which stands to save some Mac and Apple Watch owners trouble if they regularly log in and out of their system.
The technology requires that a person be wearing an "authenticated" Apple Watch when they approach a powered-on Mac sitting at its login screen, according to Apple. The computer will then transition to the desktop within a few seconds, eliminating the need to type in a password.
Authentication means that the device is not only paired with an iPhone but has been approved via a PIN code or Touch ID fingerprint recognition. People must also be wearing their Watch, since the device's sensors will break authentication if they detect being removed from a wrist. The same scheme is what allows the Watch to use Apple Pay.
Apple hasn't specified what kind of wireless protocol Auto Unlock relies on, but it's presumably Bluetooth, since that has a short enough range to be precise, and Macs lack any built-in NFC support — at least so far.
People without a Watch should still be apple to rely on third-party apps like Knock, which requires only that you have an iPhone. Apple's system is simply more convenient as there is no middle step involved.
macOS Sierra will launch as a free update sometime this fall.