The alpha track function is basically a large buffer. The cdplayer has a second pair of outputs called alpha outs. Basically it allows you to play another track of the same cd, while one track of that cd is already playing. Pitching, scratching and some other functions are still possible even on the alpha track.

Either you can mix two tunes from the same cd, or use the alpha track as a basic sampler (sample little bits of another tune on the same cd, and play them over the live track).