The 'Knock' app uses a gesture unlock system that helps users to access their locked computers simply by tapping on the screen of a phone.
The combination iPhone/Mac app makes use of a new technology that newer Apple products have — Bluetooth Low Energy. Between the two devices, a secure connection is established, like a lock and key. What's even more impressive about this technology is that your phone doesn't need to be unlocked or even running the Knock app to unlock a computer.
Since it uses Bluetooth LE technology, the app does not consume any power from iPhone batteries. Since you're already likely to have you phone on hand, this gesture unlock system seems like a much faster way to get into your computer, without having to waste time trying to remember and typing passwords.