With the app installed and running on an iPhone 4s or later iPhone, you simply only need to “knock” – no literally – on the handset twice to unlock your computer. Of course both devices will need to be in proximity) of each other for it to work, as the Knock app uses Bluetooth to communicate with the associated computer.
According to 9To5Mac, the Knock iPhone App doesn’t need to be running and on the screen, but it must be humming along in the background to work. I’ve yet to test out the app, but battery drain should be relatively minimal since it takes advantage of the iPhone’s low power Bluetooth mode.
If security is of paramount concern, it’s probably not a good idea to use Knock; if someone steals your phone or gets ahold of it, they can then access your computer. But if you’re a casual computer user, so simply want a novel way of unlocking your Macbook, then have at it.
The Knock app for iPhone (or iPad) is $3.99. The Mac app is free of charge.