Please see http://www.rockbox.org/mail/archive/rockbox-archive-2010-10/0005.shtml for the description and http://www.rockbox.org/tracker/11767 for the implementation.
My immediate impression is that it is certainly much more frequency neutral and subtle. Sometimes, I can barely hear a difference between with and without, just a slight lessening of that "dental floss stretched between the ears" feeling that I get without crossfeed. The custom crossfeed certainly intensifies the lower frequencies, and has a tendency to somewhat muffle the sound, but entire soundstage seems to move "forward" when I use it.
Despite liking bass, I think that I'm going to use the Meier version more for now, as the clarity of the music is certainly more pronounced.