Heel to Toe Shifting is when you put your right foot in a position that it can hit the break and gas quicker. By using your toes you can press the brake and you can use your heel to hit the gas. Its a racing technique that can help with downshifts and prevent you from losing speed taking corners or losing control of the vehicle.
double clutching (or rev matching) is a downshifting method that can make the ride more comfortable on downshifts as well as reduce the wear on the clutch and the vehicle.
If your car is in fourth gear and you want to go to 2nd, you know there is going to be considerable jerk. push in the clutch, put the shifter in neutral, rev the engine to the approx. transmission speed, push the clutch in, put the shifter into 2nd. If done correctly there will be NO jerking and you'll save some life on your clutch.