Vocal Inversion Slip Streaming Backmasking Reverlyricing 
Answer: Backmasking
Although the idea of hidden messages in music captured the popular imagination in the 1970s and 1980s thanks to widespread allegations by Christian groups in The United States that rock groups were hiding satanic messages in their songs, these allegations were almost never substantiated. The vast majority of allegations that messages, however strained and garbled, were in the songs can be attributed to a phenomenon known as phonetic reversal.
One of the best known examples of this is Led Zeppelin’s “Stairway to Heaven”; while the band certainly didn’t include a monologue dedicated to Satan, there is definitely a section of the song which, thanks to the phonetic reversal phenomenon, appeared as such to listeners primed by suggestion to hear such a message. These kind of examples, however, are more like a type of auditory trickery wherein the human brain, ever on the lookout for patterns, is tricked into hearing something that isn’t there. Actual secret messages, however, can be inserted into songs by use of a technique known as backmasking.
Backmasking is the process of taking a recorded audio message and inserting it, reversed, into a normal audio track such that the message can only be heard by playing the recording backwards. The technique was popularized by The Beatles who openly admitted to sneaking light hearted messages into their recordings specifically for hardcore fans to hunt for. One such example is the fade out of the song “Free as a Bird” from 1995’s The Beatles Anthology 1 album; if you listen to the fadeout reversed it says “Turned out nice again,” included as a humorous self-parody and tribute to Lennon.
In fact, despite the fact that the hysteria over satanic messages led many people to believe backmasking was a nefarious tool, the vast majority of baskmasked messages are so lighthearted as to never be confused with anything nefarious at all. The B-52’s song “Detour Thru Your Mind” features the silly backmask “I buried my parakeet in the backyard. Oh no, you’re playing the record backwards. Watch out, you might ruin your needle,” and other bands have even gone so far as to make their backmasks positive messages. The Information Society suggested to listeners to “Obey your parents. Do your homework. Winners don’t do drugs” in a backmask appearing in between songs on their “Are Friends Electric” album.
Now, before we leave the subject of backmasking, let’s add an ironic point of clarification to our earlier statement that satanic message allegations were almost entirely unsubstantiated. While there was never a substantiated case of backmasked satanic messages at the start of the moral panic over such matters, by the mid-1980s more than a few metal bands had slipped such messages into their music. Given how little attention had been paid to such matters and how little evidence there was that rock bands were employing such techniques, we can attribute the inclusion of actual satanic messages in songs to the publicity and public outrage generated by what started as nothing more than a panic in what amounts to a very odd case of art imitating life.
Image courtesy of Markus Spiske.