If you know how typical envelope filters work... well, forget them, because MXR didn't copy them. Instead, they've re-defined what an auto-wah can do to funkify your sound.
The MXR Auto Q from Jim Dunlop triggers at any playing level, not just on your loudest notes. The Auto Q's effects are amplitude independent, so you get exactly the same effects at high volume or low. Come out front with your solo then drop back for a rhythm fill your tone mods stay constant throughout.
MXR Bass Auto Q M120
With the Decay control, you can determine when the shimmer effect comes in, leaving, for example, staccato lines funky while adding a hypnotic throb to sustained parts. The Auto Q features rubberized, nonslip knobs for quick, on-the-fly adjustments.