You siu kang and go home early lah, abu den?
Seriously, if you short-shift, you change up a gear before the engine hits the optimal RPM. For example, you go up from second to third gear at 2500rpm instead of 3000.
Because you're keeping the revs lower, you're theoretically getting better volumetric efficiency, i.e. giving the fuel-air mixture time to burn more completely in the combustion chamber with the attendant increase in energy from the fuel.
However, keep in mind that short-shifting does affect the handling of the car, especially if you've got poor low-end torque - the car would obviously feel more sluggish after the gear change because it's starting out at a lower rpm.
