What's the Science Behind Ass Milk?

I've just seen numerous porn videos of girls shooting milk out of their assholes and I'd like to know, well... what's up with that? Squirting was hard enough to get my head around...

Here's an animated gif of it happening in action: http://images.heaven666.org/images/beautiful-ass-milk-fontain.gif

Oh I get it. I wonder what the point is, it's not like you can be... breast fed from an ass.