In the words of Keeley Effects:

The Keeley Neutrino V2 is an amazingly toneful and compact form of the

classic opto-coupler envelope filter. The Neutrino takes your guitar

playing and creates an automatic wah sound based on your pick attack!

Envelope filters or auto-wahs as they are often called, sound and react

best when they are opto-coupler based. The opto-couplers used in the

Neutrino are particularly smooth, giving you an amazing vowel-like tone.