This is the art of replacing the amount of braking force from the back brake, with a similar amount from the front brake without upsetting the balance of the bike. Let's say that the front brake is ten times stronger than the rear, so 1 unit of front brake application is worth 10 units of rear brake application.
We have all noticed hopw quickly the rear brake 'fades' under heavy use, so in order to have the optimum braking for the entire attack, we need to use the front brake just enough to keep the rear from becoming useless.
Top riders will tend to use just one finger on the front brake lever (usually the middle) so that they have a very high degree of brake control without running the risk of using too much and losing the front.
This is an 'advanced' technique, but can be practiced readily enough, but only with the bike travelling in a straight line to begin with.
Whizz along at a steady speed and apply enough rear brake to reduce the speed by about 10%. Slowly release the pressure on the rear brake lever whilst at the same time adding a small amount of pressure to the front. If you are doing it right there should be no variation in the speed. Try the technique at different speeds and with varying amount of rear brake pressure. Eventually you should be able to apply maximum rear brake and then almost immediately replace it with sufficient front brake. By getting this technique right you will save your rear brake from fading during an attack so it is always there when you need it.
Dedicated to the wonderful sport of Moto Gymkhana