Time-scaling, also known as time-stretching or the tempo or master tempo control, is the process of compressing or expanding the timebase of audio (speeding it up or slowing it down)—without affecting its pitch. This is very different to slowing down/speeding up in the usual sense where the pitch becomes correspondingly lower or higher and is a process that is considered state-of-the-art.
Time-scaling is a necessary capability for flexible beat mixing though there are many other reasons why an operator may wish to speed up or slow down a song in realtime without affecting its pitch (eg. karaoke, radio programming-timing reasons, influencing the "feel" of a song, etc).
Techniques to perform this feat vary considerably with all algorithms exhibiting at least some audible distortion or degradation of the underlying signal, however, the key is the perception of transparency or "cleanness" of the result.
The Ots Labs ClearScale™ time-scaling technology wins on 3 counts—its wonderful sonic clarity making it appropriate for professional use, its incredibly low CPU consumption facilitating use in live playout systems even on modest hardware, and the unmatched versatility it provides allowing slow-down right down to 0% (stationary) with floating-point precision.