This test purports to discover whether you are right- or left-brain dominant. I'm not sure whether it does, and I'm still less sure about the traits they list under right and left brain. Still, it's a lot of fun... try it.
At first I saw her turn clockwise (from the top.) With quite a bit of effort, I did learn to reverse her direction. Here's some tips I've read from people online on how to reverse her direction: use peripheral vision; look away and then look back; cover her with your hand, previsualize her turning the opposite way, then slowly uncover her; use the eye opposite to your brain side.
From 3D perspective, this test takes advantage of an orthographic view... a view without perspective. Closer objects don't appear larger, and faraway objects don't appear smaller. This allows for the confusion in silhouette.
(Via Kos, where a poll shows that about 70% see her turning clockwise at first, and 30% see her turning counter-clockwise.)