The “taptic engine” on the Apple watch pulses your wrist to signal events.
https://www.quora.com/How-does-Apple-Watchs-taptic-engine-work
It’s been used in other Apple devices like trackpads, and was miniaturized to fit in the Watch. It vibrates or pulses. On certain messages you’ll get a pulse on your wrist, instead of a ringtone or buzz that would disturb others. When I use it with GPS map functions and come towards a freeway exit or turn, it will pulse my wrist while announcing to turn left or right. Very handy so you won’t miss an exit on the freeway or a hidden driveway entrance. Especially handy for wives who keep their iPhone in their purse and won’t feel a vibrating phone, but will feel the Watch vibrate or pulse.
Oh, I like that.