A scratch is nothing but the back-cueing that you hear in your ear before you push it out to the crowd. All you have to know is mathematically how many times to scratch it and when to let it go — when certain things will enhance the record you're listening to. For instance, if you're playing a record with drums—horns would sound nice to enhance it so you get a record with horns and slip it in at certain times.

Quoted in Rap Attack 2 (1991) by David Toop, p. 65