The important thing about this tweak is that if you purchase this tweak then you will be able to get the MyStrings app for free of cost. Using MyStrings app you can customize a variety of strings such as Slide to Power Off, Slide to Unlock, Slide to Answer etc. SIGnify is developed by MarksiPodTouch and Nazaroth.
SIGnify tweak is available in Cydia under ModMyi repository. It costs $1.50 USD. In order to install this tweak you will need to have a jailbroken iPhone. You can follow the step by step instructions posted here to jailbreak your iPhone on iOS 4.2.1 with Redsn0w.