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.
