Beat-juggling, a technique used almost exclusively by hip-hop DJs and turntablists, involves using the crossfader to cut back and forth between cued portions of two different records, thereby creating a new song. First, the DJ finds a beat on record A and holds the record at that point. After releasing the record and playing the beat, he or she crossfades across to record B, on which the next beat has already been found and cued. Then the technique is repeated with record A, and so on. In this way, the DJ "juggles" the beats, using pre-existing rhythms to create one which is entirely new. This is all done incredibly fast: a good DJ can juggle 7 or 8 beats while you're reading this sentence.
The theory of beat-juggling was invented by Grandmaster Flash, who was the first DJ to cut between two different beats in time with each other. The DJ usually considered to be the first true beat-juggler, however, is Steve D., while turntablists such as DJ Shortkut of the Invisibl Skratch Piklz have taken beat-juggling to new heights of sophistication.