I was trying to get Scott Stillwell to chime in here, but for some reason it wont allow him to post.
Here's what he had to say on the matter:
"Hardware or plugins like
the SPL transient designer are NOT compressors or expanders in any traditional sense of the word.
What they do is to measure the difference between two envelope followers with differing attack/release characteristics.
For attack, we would use two followers with short and long attack times and then watch the difference in their rise as a transient hits them. That difference is a control signal to amplify (or cut) the signal.
The same with sustain...we take two envelope followers with long and short RELEASE times and use the difference between them as a control signal to boost or cut the signal FOLLOWING the transient.
It's very simple and very cool, and there's nothing else that works quite like it...it's not a compressor, nor a expander.
It's neither, and it's both."
ETA, more from Scott
"Another difference between a TD and a comp/exp...there is no threshold. It works on differential between those envelopes...which is there just as much at -100 dB as it is at 0 dB"