Kip, upon closer examination of the song file you posted, there seems to be a really fast ping-pong delay effect between opposite speakers. In other words, a sound which occurs mostly in the right speaker is repeated in the left speaker about 125 milliseconds later, and then again in the right speaker about 125 milliseconds after that. Each repeat grows weaker and weaker and it can happen either direction. I think that is how the widening effect works too, but I'm not positive.
That's my best guess anyway.