A guitar effect that does not work like a wah pedal. It does, however, work as a continually variable delay pedal. The delay on a flanger is too short to hear as an echo and the rate of delay expands and contracts to set intervals during a set amount of time according to how the operator programs it.
The effect is called "flange" because before the advent of flanger pedals, sound engineers and musicians would press the inside of the thumb (the "flange") onto the tape reel to slow it down and speed it up to achieve the desired effect.
Forget the flanger in anything by Simple Plan; just listen to Thru the Eyes of Ruby by the Smashing Pumpkins!