Wah by proximity.
This effect is a theremin-style antenna controlling a wah, getting brighter as your foot approaches. It has three extra features, however. There's a Super Hard-On boost circuit in front of the wah, with a knob to set the exact level of boost for driving your wah sound. It can be turned up high enough to make the wah crack up a little for that "something's gonna blow" sound. It gets pretty darn loud! There is also a "Mix" knob for dialing in the desired wet/dry mix of the effect, and a "Range" knob for controlling/limiting the ultra-bright upper range of this circuit.
HOW IT WORKS
The probe circuit generates a small (one or two inches high) "bubble" of RF energy at about a million cycles per second above the antenna plate. As your foot or hand (or any wet or metallic object, for that matter) approaches the copper plate, the RF field is disturbed and the circuit reacts by increasing the brightness of an LED, which drives a photoresistive cell and controls the Z. Vex wah circuit. The wah circuit is actually the Seek-Wah sound circuit without the sequencer control. The only difference is the boost circuit in this Wah Probe model.
Range: Determines how bright the wah can get. Turn this all the way clockwise for maximum brightness (this is the setting in the original Wah Probe). Turning this knob counter-clockwise makes the range of the wah akin to more traditional, vocal ranges.
Volume: This is the volume level of the "drive" knob... just set it so that when you kick on the wah it's doing its stuff at the right intensity. It goes from slightly lower than the normal guitar volume to completely bonkers when turned up all the way. It's not necessary to turn it up all the way, in fact, it seems to pretty much obliterate the wah sound, but, to each his own. Abuse those transistors, please!
Mix: Controls the wet/dry mix of the effect. Turn all the way clockwise for pure wah, or all the way counter-clockwise for just the unaffected signal. Everything in between is a mixture of the two signals. This is great for stacking with fuzz or distortion. Bass players will also like this.
There is also a trimmer on the bottom of the pedal that controls the RF field. If you turn it in the direction of the arrow (to the left) it will make the probe circuit more sensitive and set the "bottom" setting of the wah higher and higher. It's best to hold the pedal completely upside down so that the cords are dangling away from the antenna, with your hand and arm completely away from the antenna, and strum your guitar's open strings (you probably have to have it on a stand or strapped on) thru the Wah Probe. Gently adjust the RF pot with a tiny screwdriver in the direction of the arrow until you hear the wah get to the level you'd like it to be when you are not putting your foot near it. There's a region where the sound of the guitar is kind of "muffled" and the wah kicks in after your foot gets near the pedal, which is how I ship the unit. If you like the wah to be partway cocked into the lower wah region just park the adjustment right where you like it. As you get closer to the unit, it will wah higher.