Does anyone know how it works?

All the one page summaries I've seen talk about it spitting out two streams, but that's just the backwards compatibility mechanics, isn't it?

(Please excuse the following uneducated stab in the dark.)

Presumably they have a scheme that copes with high frequencies really well, but in order to maintain some backwards compatibility, they just encode the residual signal from the low bitrate mp3 using this scheme.

But I'm just curious to know (vaguely) what their scheme is.

Richard.