TubeSag originated a kind of processing that ended up in Desk, in more manageable form. What you get is a Depth control and two Speed controls, that work in parallel.
With these you dial in areas of sag. Sag works like this: it’s like power supply sag (which is what it got called in Desk). Loud sound going through the plugin for an extended period uses up ‘power supply energy’ and that attenuates the output. It’s faster than a compressor and doesn’t really have attack or release speeds, it’s just tied to the overall output.
Most applications of this will make output sound kind of grungy and degraded. Lower speeds make it seem on the attacky side, higher speeds accentuate treble edginess.
If you’d like a copy of this plugin, buy the current version of Desk and email me: I’ll send you one (but Desk does already do this, just not at two frequencies at once)