Steve nailed it. Each of the four 'programs' on the tape has two tracks, hence the term 'Eight Track'. The two tracks on each program give the stereo effect. If you catch a tape just right in its rewind configuration you can see a short section of metallic splicing tape which trips the sensors in the player head and moves the head up or down to the next program.
Find a junk tape and pop it apart sometime. The rewind system on them is kinda cool. "Back in the day" I used to salvage damaged tapes from some of my friends and rewind them. Many times they're still good even after the player 'eats them'.
