This is the non rooted version, if you're a power user, use the rooted version.
it's %50 automatic and you have to switch off the 3g/4g etc yourself (GPS can be left on always as it does not use power until an app requests it)
How it works..
It turns off non essential services!
Then it alerts you to the fact you left your mobile data on, it may be low volume in which case simply turning it up will help!
when you hear the sound you can either disable the the sounds from the app or turn off the 3g/4g connection from your settings!
This will prevent you wasting bandwith or getting charged money for data using your movile 3g data, when wifi should be on!