The purpose of the application is to switch on your screen during a call if you don’t have it against your ear. This is useful if you need to enter digits for voicemail or telephone banking, etc. The screen will be switched off if the phone is against your ear (since v1.0.10 this will include as you a making a call, as well as during the call). From v1.0.11 if you put the phone face down, it will switch off the screen, and turn on the Speakerphone (from v1.0.19 this is now an option).

http://forum.xda-developers.com/showthread.php?t=488991