No idea, but it's pretty amazing isn't it?! And largely they seem completely unfazed. I know it would certainly upset me to chuck like that! Guess it's just that the reflex is strong and the diaphragm contracts really hard, just like when we chuck. But because all their other muscles are relaxed and they're not fighting it, it just comes right out! I think the sphincter above the stomach is under-developed at that age too so there's even less resistance.

DD was a happy chucker. Big spews daily until about 6 months. One night she was asleep in her bassinet post-feed and I was just dozing off. I heard her do a funny sounding cough and thought 'oh no, I hope that's not...' grabbed a rag and lunged but I was too late. Turned just in time to see this amazing arc of milk come up, out of her mouth, over the edge of the bassinet and fountain perfectly into my underwear drawer which I had foolishly left open!! Ewwww spewy undies and bras!