I just logged into my password-protected Mac simply by rapping my knuckles twice on the screen of my iPhone. Even better, I did it in less time than it would take to type one key on the keyboard.
I achieved this bit of futuristic magic using a new app called Knock. Knock is a combo iPhone/Mac app that lets users unlock their computer, simply by tapping on the phone screen.
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Knock works by pairing your computer (a 2011 or new MacBook Air or Mac mini, a 2012 or newer iMac or the upcoming Mac Pro) with your iPhone (iPhone 4S and later). Simply install, for $3.99, the Knock iPhone app [iTunes link], install the free Mac app from the Knock website and follow a few quick steps and you're ready.
Knock works by using Bluetooth Low Energy, a technology that modern Macs and newer iPhones have. The two Knock apps create a secure connection with one another and when you "knock" on the screen of the phone, it transmits the unlock key to your Mac instantly.
Because it's Bluetooth LE, this means that you don't have to have the Knock app open on your phone to use it — in fact your phone doesn't even have to be unlocked. As long as it is powered on and Bluetooth is enabled, it works.

The guys at Knock say that because of Bluetooth LE, it won't suck any power off your battery. In my limited tests, I found this to be accurate.
You can be within the Bluetooth range of the two devices, about 20 feet or so without obstructions.
The Knock Mac app runs in the background and it integrates seamlessly with the standard Mac login screen. A green circle surrounds your login avatar, letting you know Knock is available. Then just tap twice on your phone screen and voila ... like magic, your Mac is unlocked.

Knock was created by William Henderson and Jon Schlossberg. The two came up with the idea after realizing how much they both hated having to constantly unlock their computers.
As with phones, about 50% of users don't bother with passwords — and those that do are often pretty terrible. Combinations like "1234" that are easy to type.
Before Knock, Henderson worked at Square, where he created Square Wallet. One of Square Wallet's big features is that users can pay just by saying their name. That type of human approach to security is something the two want to carry on with Knock.
I've been using Knock for most of the day and I've found that it is much faster to unlock my computer using it, rather than typing in my password. This is a password I type in dozens of times a day — and one that I've used as my login password for more than a decade. Still, hitting my knuckles against the phone is even faster than moving my keys across the keyboard.

Really though, the target market for Knock is users who don't bother with a password (or a good password) at all. This is where the design of the app really comes into play.
And what's more, because Knock integrates so completely with the Mac login system, if you misplace your phone — or it's out of battery — you can still enter in your password to login just as you normally would.
OK, so what's the catch? Well, Knock doesn't work all the time. If you restart your computer, Knock won't work until you've logged in once. The program that runs in the background needs to be running.
Likewise, if you switch from one user account to another, Knock won't work until you're logged back in to the user account it's paired with.
If your phone is lost or stolen — you can disconnect it from Knock by clicking the Knock icon in your menubar and selecting "Unlink [iPhone name]."
So what happens if your Mac and your iPhone are both stolen? Well, the good news is that the thief probably won't realize or necessarily know to check that Knock is installed, thus making getting in less likely. Also, keep in mind that you can remotely wipe your phone and Mac using the Find My iPhone and Find My Mac features.
If your computer were stolen while turned off, Knock won't work until you've logged in with a password on that computer.
I initially decided to play with Knock as a novelty. I mean, who doesn't love the idea of knocking on their phone to unlock their computer — especially from across a room.
What I've found, however, is that Knock represents a really smart way of handling security. It's faster than entering in a password and it transmits data in a secure, smart way.

I'd really like to see technology similar to Knock paired with other types of devices and technology. We talk a lot about the "Internet of things" (a term I personally hate), but we rarely look at experiences that can make it easier to follow good security practices.
Like Touch ID, Knock is one of those apps that really gives the average user no excuse not to use a password.
Plus, I won't lie, the parlor trick of knocking on my phone to unlock my Mac is pretty slick.
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Images: Knock; Screenshots by Mashable