Flanging first came into popularity in the 1960's. The effect was produced by slowing down playback of a recorded track by dragging a finger against the tape-reel "flange" to delay the signal. When this altered track was played back against the original the resulting peaks and notches in the frequency spectrum produced the signature "swoosh" we now know so well. Later the effect was duplicated using analog bucket brigade IC's and now, of course, with digital electronics. However, nothing has quite captured the sound of those old tape reels being slowed down and sped up.
FL-3 FLANGER page at Guyatone 's website...