When you hit a jump normally, your suspension releases at the top of the lip. With a scrub, you release the suspension sooner in order to stay low. According to Bryn Atkinson, the key to the scrub is modifying your approach to hitting a jump. The first step is to go fast, up to two times faster than you would normally approach a lip. As you start coming up the face, you initiate a corner. How hard you corner will depend on the steepness and length of the face. Once you hit the lip, you start cutting back. The idea is to change the point where the suspension is preloaded off the face of the jump. After you’re in the air, Atkinson suggests you treat the scrub like a normal jump. Your goal should be to straighten the bike out, aim for the backside, and ride it out. To learn more check out this video from the Global Mountain Bike Network.
The scrub is a fairly advanced technique, so if this sounds too complicated, best to start with the basics of jumping and check out this list of tips and tricks.