Stereotypically, concertgoers to a heavy metal concert will be doing two things: violently throwing their heads back and forward in time with the strumming of power chords on the guitar, and making a hand gesture that is iconic - a fist with the little and index fingers extended, with the thumb sometimes extended as well. The hand gesture is referred to as the devil horns, metal sign, named with a countless number of other terms. It's a hand gesture with a long history, which I'll get to in a moment. It does NOT derive from the American Sign Language for "I Love You". At your typical concert, it is typically raised aloft with the right hand and "pumped" in rhythm with the music, or shaken violently upraised at arm's length.
What's lesser known is how the symbol came into common usage in heavy metal.
There are a few claims to the gesture: Gene Simmons claims to have invented it and used it on the cover of the 1977 album Love Gun. The setup of the hand was previously seen on the illustration of Yellow Submarine by The Beatles, so if simply having your hand in that shape on an album cover represents its invention, Gene stole it from John Lennon.
However, second Black Sabbath singer Ronnie James Dio popularized the gesture at Sabbath concerts, and the gesture struck. Dio, upon joining the band, noted that Ozzy Osbourne was famous for tottering around the stage with the "peace" symbol upraised, index and middle fingers making a "V" shape. He liked the idea of the theatricality of it, but didn't want to copy the singer he was replacing.
He did, however, wonder just why in hell there were crucifixes in their stage shows, on their person, and inlaid into the fretboard of Tony Iommi. Truth be told, they started wearing crucifixes after being "cursed" by a group of self-described Satanists who approached them to play a Black Mass, when they told their visitors that Black Sabbath was no more Satanic than Vincent Price, they were entertainers.... Iommi went home that night and made them all crosses to wear for protection. Ozzy still wears his today, as do the rest of them.
Dio is of Sicilian-Italian heritage, and remembered his grandmother's use of the corne to ward off evil. That was its original use, as an anti-black magic protection gesture. So in a flash of serendipitous imagination, he decided to raise his hands in the protective horns gesture in concert. And whether he was the first to EVER do it or not, his theatrical flourish of the "double viking" made it the "de rigeur" heavy metal fan affectation with that certain soupcon, that certain elan.
Of course, evangelist groups point to the Beatles, KISS (Knights in Satan's Service!), Dio and Black Sabbath, and the explicitly Satanic imagery of Slayer and claim a demonic connection in rock music. But what it ultimately boils down to is that Ronnie happened to come up with the right flash of inspiration at the right time, and the rest is rock music history.