Q:

How are backward messages created on tracks?

It is famously known that on Led Zeppelin's "Stairway to Heaven" are hidden satanic messages if you play the track backwards.
Is this true? I've heard some backwards recordings on Youtube, but I don't know how true that is.
If this is true, how are backwards messages like these created?

A:

I would say this is "famously claimed" :-) -- I'd be surprised if this was found to have been intentional.
But of course, the rumor of Stairway's hidden message spread throughout popular culture, and since the technique to actually execute something like this is relatively simple, it has  certainly occurred in media since then.
Hiding a message in an audio recording in this manner requires only simple tape manipulation techniques of overdubbing and tape reversal. You record your message on one tape, play it back in reverse, and while doing so, overdub it on top of your song.
After achieving this on a basic level, you can add complexity to the process by effecting the message recording, and dubbing it over a part of the song that is particularly busy, so that it becomes "hidden" in the texture of the final song -- making it less obvious what you're doing when you listen normally, but still clear when playing in reverse.

A substantially different method would be to write your lyrics in such a way that, when singing them with a very particular phrasing, you get a result that when played backwards starts to sound like a different set of words. This might be accomplished by starting with your "hidden message", listening to it playing backwards, and then trying to figure out what forward-sounding lyrics sound the most similar to the backwards message.

