I have understood how flanging (and the related phasing effects) works for many years, and have emulated the old techniques many times with modern gear, but this week I had a chance to hear the classic flanging effects in action (while using modern tools). This will be less confusing when you watch the video.
The term flanging comes from tough the flange, the metal spool for a real of tape, to slow the speed of a tape machine. You can learn about the origin of Flanging on this Wikipedia page.
I did the same thing couple of years ago where I pulled a processed mastered track and played it together with original pre-mastered track.
Listening to the phasing effect was really nifty!!! I bet it was mostly coming from the EQ’s phase response vs the original!