AUTO-WAHS (or envelope filters) work differently from a traditional wah pedal in that they don’t use a rocker pedal to control the sound.
Instead, they are controlled by your playing dynamics: how hard or softly you hit the strings. Essentially, you set a sensitivity level for the effect, and once your picking reaches that level, the wah effect is triggered. Most auto-wahs let you decide the sweep’s direction (low to high, or high to low) and sweep speed (how quickly it rises and how quickly it falls).
These controls let you create extremely fast movements, so it’s possible to create sounds with an auto-wah that you’d find difficult with a regular pedal. Some auto-wahs, such as the Boss AW-3 Dynamic Wah, use a low-frequency oscillator (LFO, controlled by the rate knob) like you’d find in a modulation pedal to control the sweep of the wah – this gives you consistent rhythmic sounds without tiring out your foot!