Not much has been said about the circuit from the creator who just simply links to the schematic on his website, but here is a great descriptions from Floris:
The Flatline compressor is basically a non-inverting amplifier around a opamp. The optocoupler together with the gain resistors determine the total gain of the circuit. The idea of the compressor is to have some gain (typically 20x) when there is no or a soft signal present and to have a unity gain when a large signal is present. So in the guitar attack phase (a large signal) , there is (almost) no gain and in the release phase, the gain increases to 20x while the guitar signal is fading out. This gives the impression of a compressed sound => Less volume dynamics. It also increases the noise level with 20x (=26dB) which is not so nice but inherent to this design. There is a “sustain” pot that is used, as part of the gain resistors, to change the gain difference (=gain ratio) between loud and soft signals. With minimum sustain, the gain ratio is about 3x. With maximum sustain, the gain ratio is about 20x.
You can find a lot more information about the circuit on Floris’s website.
As an option you could replace the 47K resistor with a pot to give you a “Sensitivity” control.