I know it's got something to do with a wah, and I also now that it makes some freaky sounds:P
A hero of war, Yeah that's what I'll be

Quote by pak1351
I know it's called auto-wah these days, and people bitch about it on their valvetronix's

Actually, auto wahs are time based.

Envelope filters are controlled by playing dynamics (the amplitudes of the electric signal going into it).

And envelopes can be used with other effects, so that they get applied in different ways at different dynamic levels. You could possibly use one with a whammy for example. The louder you play, the more pitch shifted you get. Don't think anyone does it, but it is an example. Envelope filters are just the most common kind.
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Quote by elliott FTW
Damn you and Warwickyness

Quote by ScottB
gm jack knows everything
Auto-wah (also known as an "envelope following filter" or just "envelope filter")

1) A typical auto-wah circuit uses an envelope follower to produce a voltage representing the overall volume of the input. This signal is then used to sweep the cutoff frequency of a filter. The filter usually has a low pass or bandpass response. The Boss AW-3 is an example of such a device.

2) There is a variant of auto-wah that utilises a low frequency oscillator (LFO) instead of an envelope follower to alter the effect. The filter response varies constantly with time and is not linked to playing dynamics. The Boss AW-2 is an example of such a device. The difference in sound is subtle, but careful listeners will notice the constant period of the filter sweep. The song "Falling Into Grace" by The Red Hot Chili Peppers is a good example of an LFO controlled auto-wah applied to the bass guitar.

Same effect