A low-pass filter swept by its own LFO. It gives any sound a rhythmic, wah-like movement that keeps time with its own clock rather than with what you play.
The idea
The filter's cutoff swings around a centre frequency on an exponential curve, so each step of the sweep covers the same musical interval whether it sits low in the bass or up in the treble. The low-pass is maximally flat, with no resonant peak, so the sweep shapes the tone without making the signal louder than it came in.
Because the LFO runs freely, the effect does not follow your dynamics. That is the difference from an envelope-driven auto-wah: the movement stays steady whether you play softly or hard.
