Face melting is what happens to the crowd at a hard rock/heavy metal concert. Its a by product of "the rock" and "rocking out" usually brought about by a nasty guitar solo or killer breakdown. Can include other facets of heavy metal music, like "high voltage", "explosives", "damnation" or the like. Also heard in the iteration, "you melted my face." Bands known for face melting are AC/DC, Iron Maiden and Judas Priest, however Rush has been known to melt faces, and Eddie Van Halen of Van Halen is reported to have an effect pedal that generates "face melting" sound. The only real case of (out of context) face melting on record is from "Indiana Jones and the Raiders of the Lost Ark" where at the end, Nazis get melted. Also, the "World of Warcraft" community has borrowed the term for a particular sequence in their video game.
Malcolm Young says to Angus Young, "Lets get out there and commence with the face melting!"