the feature on many mechanical movement watches that stops the second hand when you pull the crown all the way out to set the time. This makes it much easier to set a mechanical watch precisely to the second when synching with a time signal or known accurate clock. Older watches less commonly have this feature. To simulate the hack feature on many watches that don't have it, pull the crown out to the time setting position, then try gently turning the crown backwards. This puts a small amount of back pressure on the watch movement, which may stop the second hand long enough for you to synchronize it with another clock.