Here's what Cornerstone say about the Zefiro:
The Zefiro Optical compressor has been designed for people who don’t like compressors, but instead want the tonal-shaping and sustaining behaviour of a compressor. With this design purpose, the Zefiro features a very smooth and soft action, much like a studio compressor, together with an extreme tonal transparency. It is very useful for adding warmth and balance to the sound of electric, acoustic and bass guitars.
Looking at the controls, there are Comp, Volume, Tone and Sensitivity. The Volume control acts as a master output level control. The Tone knob is a tilt-eq control and changes both the bass and treble frequency response at the same time: the more treble the less bass and vice versa. This tilt-eq operation has a pivot frequency set at 1kHz, in the middle of the guitar audio frequency spectrum. When the Volume and Tone controls are at noon, there’s no loss in volume and the eq is totally flat.
The Comp control determines the amount of compression, which can vary from very subtle up to quite squashing. The Sensitivity control acts much like a threshold control; it changes the compression level and so manages the overall dynamic response, still keeping the same amount of compression.
The Zefiro requires a standard 9V DC Negative power supply only.