The principle of the DDT is to get sideways and hold the opposite shoulder button to add the strafe speed to your total speed. So if you're steering to the right and hold L you will strafe in the direction of travel and the top speed will be higher. Hold the drift as long as you can then release L and steer left and hold R. Repeat this until you run out of track! Note this is only for use on long straights and you will need to practice how often you can do this before you hit the corner.
As a previous poster mentioned, you should master strafing first, hold the shoulder button and steer slightly to correct and your top speed will increase (although the speedometer does not reflect it). When you do this you get the additional speed of the strafe, but not all of it as not all of it is in the direction of travel. The DDT allows you to get almost all the strafe speed.
Source: I spent most of my adolescence playing this game.