In the words of Electro-Harmonix:

Envelope controlled filter is an effect type that ventures beyond the mainstream yet is equally at home in a wide range of genres and equally effective on guitar, bass, keys, horns and more. Electro-Harmonix is a pioneer in this effect category and has been making envelope controlled filter pedals since the 1970s! The Nano Q-Tron springboards off that deep heritage.

Heres how it works: The Nano Q-Tron responds to the users playing style and dynamics. The volume of the input signal it receives (also called the envelope) controls the cutoff or center frequency of a swept filter. This allows the Nano Q-Tron to create tone-altering effects ranging from subtle to dynamic, auto-wah to ultra-funky. Its all about how you play and its super interactive.