The CS-1 is a direct successor to the Roland AS-1. And like the AS-1 it does its job by means of photocouplers - different to the later CS-2 and CS-3, which are using VCA's (Voltage Controlled Amplifier) to create the same effect. Like all compression pedals it reduces the output level over a set threshold relative to the strength of the input signal, making levels consistent and enlarging sustain.