Most guitar compressors are based on OTA circuitry, but T-Rex's NeoComp uses a VCA (as does the Boss CS-3), and is apparently modelled on the studio-grade Blackmer VCA.
Comp and gain knobs control the amount of compression and the output volume, with the sound's envelope shaped by attack and release knobs.
The NeoComp is capable of light compression up to a quite heavy squashing of your signal. The drop in volume can be compensated for with a gain knob that has plenty in reserve, and can really drive your amp if you want to use the pedal as a booster.
Juxtaposition of the two knobs can dial in a nice boost with a little compression; and where some compressors leave you stuck with a note envelope that may not suit you, the NeoComp lets you dial in the snap of the note with real accuracy, using those attack and release knobs.
The NeoComp delivers an adjustable range of practical compression.