Leave your PC - it's locked automatically, come back and it's back too... This software helps you add a little more security to your desktop. It does so by detecting one of your bluetooth devices, most likely your mobile phone, and keeping track of its distance. If you move away from your computer and the distance is above a certain level (no measurement in meters is possible) for a given time, it automatically locks your desktop (or starts any other shell command you want).
Once away your computer awaits its master back - if you are nearer than a given level for a set time your computer unlocks magically without any interaction (or starts any other shell command you want).