Great, thanks. It seems there is no technical description available anywhere beyond what you quoted. They haven't written it up, and I couldn't even find comments in the source providing details. That's too bad, but he (I assume that's Jon Sneyers) does say he hopes to write it up later.

Also, comments in that thread on speed [1]:

     In terms of encode/decode speed: both are slow and not very optimized
     at the moment (no assembler code etc, just C++ code). A median file
     took 3 seconds to encode (1 second for a p25 file, 6 seconds for a p75
     file), which is slower than most other algorithms: WebP took slightly
     less than a second for a median file (0.5s for p25, 2s for p75), PNG
     and JPEG2000 took about half a second. It's not that bad though: BPG
     took 9 seconds on a median file (2.5s for p25, 25s for p75), and
     brute-force pngcrushing took something like 15 seconds on a median
     file (6s for p25, over 30s for p75), so at least it's already better
     than that.

     Decode speed to restore the full lossless image and write it as a png
     is not so good: about 0.75s for a median file, 0.25s for a p25 file,
     1.5s for a p75 file. That's roughly 3 to 5 times slower than the other
     algorithms. However, decoding a partial (lossy) file is much faster
     than decoding everything, so in a progressive decoding scenario, the
     difference would not be huge.

[1]: https://boards.openpandora.org/topic/18485-free-lossless-ima...