Activator, the famous tweak of cydia. If you are new to jailbreaking you must install this tweak immediately and without hesitating.
This tweak allows you to define an action to a specific gesture, for example : open an application by simultaneously pressing both buttons Volume or, plug your headphones to start the music automatically ... And many many more actions. It has no limits. Activator is devloped by Ryan Petrich. Repo : http://rpetri.ch/repo/