Voices as instruments

I have always wondered how the song 'I'm not in love' by 10CC was made. It became one of the top 5 songs of my youth, partly because the sound of the song was mesmerizing. That backing choral sound that seemed out of this world. How did they do that?

Well, here's the story. The boys basically recorded a bunch of vocal harmonies and played them back like a voice synthesizer, but using the mixing board sliders to play the notes. Incredible. The entire song follows the story in this clip.