It uses a version of the Droste Effect, which is based on an uncompleted lithograph that Escher made in 1956. (The name comes from Dutch cocoa brand Droste, whose packaging featured a nurse holding a tray with a packet of Droste, on which there was an image of a nurse holding a tray with a packet of Droste, on which...)
To realise the promo, OneInThree adapted some software created by Josh Sommers, and borrowed seven extra Macs from their nearest and dearest. They now had nine computers, and ran them 24 hours a day for five days, working in shifts to make sure it ran as smoothly as possible. (They still had over 400 crashes during the 1080 computer hours, and ended up with 2 terabytes of data.)
Pleasingly, the track's pretty great too.