“Uses the signal amplitude envelope to control one or more effects. The envelope follower takes an audio input and converts it into a “control voltage” which rises and falls with the volume of the input signal. The standard controls are threshold, filter, and volume. One effect which uses an envelope follower is the “auto-wah”, which automatically produces a “wah” effect based on the dynamics of the notes being played.”
Here is information on the other two earlier versions — V1 and V2.
More information here: