In a common rail diesel, the engine is controlled via a ECU (electronic control unit). The ECU controls all the fuelling related parameters such as quantity, pressure and advance (these are just a few of the many parameters, but the ones that mainly contribute to the torque output). In case of a VGT (like the Verna), it also controls the turbo. Most OEs add limitations to the fuelling parameters for various reasons and a tuning box (Such as Pete's ) helps by removing some of these restrictions to help get better performance from your engine.
Hope this helps!
