Knock uses a combination of an iPhone app (currently offered at an introductory price of $3.99) and a Mac app (available for free at Knock’s website) to enable unlocking of your Mac, wirelessly, by just knocking the back of your iPhone.
It’s a surreal experience. After a couple of minutes of initial setup, you lock your Mac and rap the back of your paired iPhone twice. Your Mac unlocks. The app doesn’t even have to be forefront on your phone, although it does have to be ‘open’ in the multitasking tray. In fact, your phone doesn’t even have to be unlocked. When it senses the Mac is near, a notification appears on the lock screen instructing you to knock. There are some nice UI touches too. For instance, echoes of the ‘sound wave’ appear on OS X’s lock screen in real time as you knock. It’s a subtle visual indicator that the system is actually working.