This effect is a natural continuation of the probe series... the Fuzz Factory probe, the volume control/manual tremolo probe, and now a wah. It's probably exactly what you think it 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.
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 ProbeTM). 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 bon- kers when turned up all the way. I personally don't think it's 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 play- ers will also like this.