Jailbreakers should be familiar with Activator because it is automatically installed and it's there when you look up in Settings. If it's not, just search for Activator in Cydia and install the package already.
Once installed, you can set Activator to use the volume button for example to do pretty much everything from unlocking the home screen, opening messages, restart the phone or launch any of your apps. The possibility is endless.
For example you can set double tap on the home screen to launch the camera app very quickly without unlocking the phone which I find really-really useful.
There are a lot more ways to customize Activator like double clicking or holding any of the buttons or using a myriad of different gestures on the home screen. You can even set the Activator to do something just by shaking the iPhone or holding the volume button for a few seconds.
Activator is an extremely powerful app launcher and control mechanism for your iPhone and especially useful when any of your iPhone's buttons turns faulty. This free package should be installed automatically with Cydia when you jailbreak your iPhone. Else you'll just have to curse your luck and go to the nearest mobile phone shop and get your iPhone fixed. Head on to iClarified.com to learn how to jailbreak your iDevices.