I couldn't tell you, other than instead of the compressor looking at the input, it looks at the output.
Not many compressors are feedback, so I think there are very few feedback comp plugs. Steinberg's emulation of the Neve Portico 5043 is feedback comp, and it's price just *dropped* to $250.
TDR isn't perfect, but it's solid, and many people love it. I did a quick shootout with it on a current mix on the 2buss and it sounded good, but it lost out to my Voxengo Polysquasher. But if I spent time to really get to know TDR, I could see it being a go-to plugin.
It's not quality for a free plugin, but quality regardless of price. Many devs would sell it for $59-$99 and no one would even bat an eyelid. It's a gem, grab it.
I'm a big fan of Voxengo's compressors. While most of them can handle duty on/for just about anything, each one is tailored for particular tasks.
The cheapest of the bunch is the Crunchessor, which I also happen to find to be the most versatile. Their Polysquasher imo is the best mixbus compressor of the one's I tried, and though I don't use it their Elephant is considered to be one of the best limiters out there.