Portal 2 walkthrough - Chapter 4: The Surprise - Test Chamber 21 (Escape)

[iGamr]

This vid helped me learn something about this map. If you pause it at the EXACT moment when the lights go off, you see Chell floating off-map (just watching it makes it look like an electrical spark). I went to the map and found out why. The unlit chamber with the light bridge disabled is actually a separate part of the map from the lit, active chamber, and when you reach a certain point it jumps you onto the corresponding point on the other map and spawns the cube... That's so cool!