Joe Rossignol, MacRumors, writing about Jeremy Horwitz’ tweet:
Starting in watchOS 6, the Apple Watch has become a trusted device for Apple ID authentication purposes.
When you or someone else signs in to your Apple ID on a new device or browser, the Apple Watch will automatically alert you, complete with an approximate location of the person. If the sign-in attempt is allowed, a six-digit verification code will then appear to be entered on the new device or browser.
Apple, firing on all cylinders.