The flanging effect mixes two identical audio signals. It applies a slight delay to one signal and continuously varies the delay time. Flanging combines two identical audio waveforms with a time delay between them. This then makes an interference pattern where the volume level of some frequencies
Visually this appears as a series of peaks and notches like a comb. They call this “comb filtering”. Since the delay time is being continuously varied, we get a swept comb filter. We hear the continuous change in the timbre that we recognize as flanging.