All we know is that it is somehow connected to the rainbows that appear whenever somepony arrives in this world... they always accompany somepony new arriving, even if it is a single pony.

[She's also technically not the one to ask about that, for an... assortment of reasons. It's far more something to ask Celestia. All she herself has is her speculations based on a slightly... wider view of the circumstances.]

I am certain that you will find that magic very rarely follows rules observable by the naked eye, however.