From MXR;

Tame your hissing high-gain amp or silence your string of stompboxes
with this ingenious pedal. The Smart Gate is equipped with three
selectable types of noise reduction to handle virtually any type of
signal. It bites down on sizzle and hum but lets the smallest detail of
your playing through. With its uncanny ability to sense precisely
when—and how fast—to engage, this little genius will never get in your
way, so you can wring every last bit of sustain out of that chord
without being cut off. The M135 features a hardwire bypass, precise
threshold trigger, and amazingly clean circuitry.