Spinning girl optical illusion

One of the most impressive optical illusions I’ve seen in a while. Which way is she rotating? She started off clockwise for me, but is now firmly anti-clockwise and won’t change back for more than a second or two. I managed it by looking at the edge of the screen, rather than concentrating on her shadow, but ymmv…

Update: Re-reading the above, I see I didn’t really express how amazing this illusion is. Usually with ‘switching’ illusions I can see how it works – that is, which part of my visual system is getting confused, and how the image is constructed so as to create the effect – but this one’s a mystery to me. It’s completely unintuitive. I haven’t had a good dig around the site yet; I got stuck on this one and haven’t progressed further. Anybody?