He heard the music in his head and then conducted it in silence on video. Hundreds of singers all over the world sang to his conducting, also on video, and he put them together into a virtual choir.
I want to steal this idea. I love it so much. As the music began playing, and the different faces of the singers began to flash, each in their own little window, I actually got a little teary-eyed. The first video tells how he did it. The second video is the song itself.