It’s called Backmasking. A process where you essentially take a recorded message and convert it to be heard in the reverse order. Probably the best example of this is in the Beatles’s song Revolution Number 9. Reverse the record on the turntable (record player for those under 30) and you’d allegedly hear the phrase, "Paul is a dead man, miss him, miss him, miss him."
After the Fab Four used it on other albums, other bands followed suit. And soon all the headliners like Pink Floyd, Queen and Styx were (rumored) to be using it quite a bit. Which then got the right-wingers so crazy that they had Arkansas legislation pass a Backmasking law in 1983.
It stated that all bands had to provide a warning label on their album covers if backmasking was present. Of course, once the government got involved it meant backmasking was too mainstream. So since it was out of subculture most bands simply got bored with the whole thing. Eventually it became as passé as the electric keyboard guitar or the whammy bar.
Today Backmasking comes in drips and draps. Probably 'cause most bands couldn't give two sh-ts about hiding their subversive lyrics anymore.
FYI...Here are some of the top past and present satanic/odd songs and phrases…
Artist: Ash
Album: Evil Eye (2004)
Backwards Phrase: "She's giving me the evil eye, suck Satan's c*ck."
Artist: Deep Purple
Album: Stormbringer (1974)
Backwards Phrase: "The c*cksucker, motherf*cker, stormbringer!" 
Artist: The Doors
Song: Break on Through (1967)
Backwards Phrase: "I am Satan"
Artist: Insane Clown Posse
Song: Echo Side (1999)
Backwards Phrase: "F-ck the Devil! F-ck that sh-t! We believe in life legit. If you diggin' what we say, why you throw your soul away?"
AND THE MOST F’D UP ONE OF THEM ALL…
Artist: Information Society
Song: Are Friends Electric (1997)
Backwards Phrase: "Obey your parents. Do your homework. Winners don't do drugs."
