On some records (vinyl that is), there is a track that plays forever, or at least until you stop the record. This is called a locked groove. This groove is can completely separate from the main groove, or it can be a continuation of the same. The music in this groove must be one of several tempos.
T (in bpm) = b beats per measure * (V rpm) * n measures/revolution
For most techno-type stuff you'll just have one measure of 4 beats at 33 1/3 rpm at a tempo of 133 1/3 bpm. Any more than one measure and you've got some crazy gabber nonsense.
Credit for the original locked groove (in techno, anyway) can be given to Ron "Motown" Murphy of National Sound Corporation (now Sound Enterprises) who tried doing it for X-102's Rings of Saturn. Since then, loops like these have been showing up on many techno releases. The User will often include a loop as its own track. Sometimes a regular cut will end in a locked groove, which throws off anyone listening but not quite paying attention.
Locked grooves are usually a novelty with one or two on a side, but Speedy J's album loudboxer consists of two discs, each with FIFTY locked grooves per side!