Most wah pedals have to be activated before the effect does anything. Often, this means pressing down the toe end of the pedal to depress a footswitch similar to the ones seen on many other types of guitar pedals.
Once on, moving the foot pedal back and forth will create a variable filter. Moving the pedal slowly creates a more sweeping, epic effect. Moving it faster results in shorter waves, which can sound percussive or vocal.
Auto-wah pedals, such as the Mu-Tron III and Boss AW-2, activate a wah effect without the need for an expression pedal. Normally, the filter is triggered by the input level. Auto-wahs first found common usage among disco musicians.