The app, which uses Bluetooth low energy technology, lets iPhones communicate with their Mac brethren when they are in proximity to one another. The Knock app turns two steady knocks on the front or back of your phone into a secure unlocking tool to let users bypass the Mac password screen.

The iOS app costs $3.99 and works in association with a free Mac counterpart. If you’re looking to buy the app, note that Knock only works on selected devices, all of which are listed below:

iPhone 4S or later MacBook Airs 2011 Mac Minis or newer MacBook Pros 2012 iMacs or newer Mac Pros from this year only.

The app does sometimes fail to work according to those who have already tried it, and it must be running in the background on your iPhone at all times. However, fortunately the phone does not require to be unlocked to register the knocking.

Co-founder William Henderson says on the Knock Web site. Henderson is a former Square employee who worked on the payment company’s mobile-wallet product. So William Henderson teamed up with Jon Schlossbery,  a former user experience designer at e-commerce company Bonobos. They founded Knock, and brought the same kind of smooth functionality Square Wallet employs to the act of password unlocking.

To showcase both the power and humor behind hitting one’s expensive piece of glass to bypass a password, Knock released the following promotional video: http://www.youtube.com/watch?v=CyX8FfSKg04 In terms of real-world utility, this isn’t that much useful. Typing a password into a Mac isn’t quite what you’d call a huge chore, although to be fair, I’m not in a setting that necessitates unlocking my machine numerous times a day. Regardless, as it stands, if an malicious individual manages to steal both your iPhone and your Mac, all security is foregone (as the iPhone can unlock the Mac without needing to be unlocked itself). Knock for iOS can be downloaded from the App Store for $3.99. The accompanying Mac app can be downloaded from the Knock website for free.