It's called the Crystal Arch, the last remnant of something called Anu. It always hums, a sort of harmony that I can't really describe except that it's very beautiful even when it was under duress.
Anyway, when a new Angel needs to be made, the other angels tune into that harmony, making something called 'Lightsong'. Then, according to what Tyrael said, luminous bands will appear and synchronize with the music, then making a new angel. But only if there's perfect harmony within the High Heavens. He also said the energy is finite, that there can only be so many angels.
It's quite interesting, I was very curious to see how it was done but they wouldn't let me see.