So the wah-wah is a very cool effect that was pretty much pioneered by Jimi Hendrix. It is an envelope filter-style effect. Without going too technical the wah; the actual pedal opens and closes the filter and the amount or depth of that filter within the signal. It's a very distinctive sound. Right here I have a very simple wah-wah pedal. It's not boutique-e. It works. It's the Dunlop Cry Baby pedal. No bells and whistles. Just a simple on/off switch.
A really cool way to use the wah-wah in a rhythmic sense, look no further than the '70's where you had a lot of funk music and a lot of soul music really utilizing the wah-wah pedal. Basically, where you're moving your foot, in conjunction with the rhythm, against the rhythm but in time, to really open up the envelope. And here's a demonstration of something that you might play if you were living in the 1970's with a wah-wah pedal. So you can see that I'm really reacting to the rhythm, opening up the filter with the wah-wah pedal. They're really fun to play with.
A really cool thing to do with the wah-wah pedal is add distortion and you can really get some really cool lead sounds. Not necessarily opening and closing the envelope filter with the pedal based on any rhythm, but more based on melodic phrases you may do to really enhance a certain frequencies that you want to bring out in the notes that you're playing. And really just feel whatever the music, however it's speaking to you, whatever you're trying to accomplish, you can really pull out some cool effects with the wah-wah pedal.