Schematic link: http://www.geofex.com/FX_images/orgsqz.gif
This is a very popular and well received compressor effect. Jeff "Skunk" Baxter can be seen with it sticking out of his Tele during the 1970s (check out live video of "Takin' it to the Streets"... he also appears to be playing through a Roland System 700 modular synth!).
This is a great example of a FET attenuator circuit. There is an op amp to provide a generous amount of gain for quiet signals, and then the input is attenuated as the output signal increases. This is like a mini version of the famous UREI FET attenuator found in the 1176.