Reverse echo or reverse reverb, also known as backwards echo and reverse regeneration, is a sound effect created as the result of recording an echo or delayed signal of an audio recording played backwards. The original recording is then played forwards accompanied by the recording of the echo or delayed signal which now precedes the original signal.
During one session [with The Yardbirds], we were recording "Ten Little Indians", which was an extremely silly song that featured a truly awful brass arrangement. In fact, the whole track sounded terrible. In a desperate attempt to salvage it, I hit upon an idea. I said, "Look, turn the tape over and employ the echo for the brass on a spare track. Then turn it back over and we'll get the echo preceding the signal." The result was very interesting -- it made the track sound like it was going backwards.
Later, when we recorded "You Shook Me", I told the engineer, Glyn Johns, that I wanted to use backwards echo on the end. He said, "Jimmy, it can't be done". I said "Yes, it can. I've already done it." Then he began arguing, so I said, "Look, I'm the producer. I'm going to tell you what to do, and just do it." So he grudgingly did everything I told him to, and when we were finished he started refusing to push the fader up so I could hear the result. Finally, I had to scream, "Push the bloody fader up!" And lo and behold, the effect worked perfectly.
Despite Page's claims, an earlier example of the effect can distinctly be heard towards the end of the 1966 Lee Mallory single "That's the Way It's Gonna Be", produced by Curt Boettcher.