This song is an adaptation of a 1961 classical piece called "Toccata Concertata" by the Argentine composer Alberto Ginastera (1916 - 1983). It was arranged by Emerson, Lake & Palmer by their keyboard player, Keith Emerson, who had a keen interest in classical music.
Emerson first heard "Toccata Concertata" in 1969 when he was playing a concert with his band The Nice in a bill with the Los Angeles Philharmonic, which performed the piece. He worked up a new arrangement, and after Emerson, Lake & Palmer formed, they decided to record it. The song ended up being a lynchpin of their fourth album, Brain Salad Surgery, and a live favorite.
Getting permission to use "Toccata Concertata" was a challenge. Keith Emerson tried doing it through the song's publishers, but that didn't fly, so he arranged to meet Alberto Ginastera at his home in Switzerland. According to Emerson, when he played the ELP version for Ginastera, the composer was delighted and enthusiastically granted permission.
This is the first rock song to use electronic drums. With the help of an electronics expert the group had on hand, Carl Palmer created a system where his drum kit would trigger programmed sounds. "In the middle you hear all these atmospheric preprogrammed sort of keyboard sounds," he said in his Songfacts interview. "But they're actually all being triggered by the drums with a mitigator on the floor where I could have an octave divided, change the sound up or down to each individual sound. And there were roughly eight of them. So those eight, 16, probably 24 sounds, the same sound, up the octave and down the octave.
So that was my first and my main sort of contribution, really. It never got picked up on because it was so far left - it was so far in front that people thought they were keyboard sounds. So we didn't really bother in explaining that, No, they're not. We just let it go, because we were just after creating new sounds and things."
The word "Toccata" means a keyboard composition designed to exhibit the performer's technique. A good fit for Keith Emerson.