I'm not sure I buy that this test measures anything substantial, but it's fun to look at and play with anyway. The trick to changing the dirction of rotation (for me anyway) is to focus on either the bottom foot or the head, and take "notice" of the rest of the body when the focussed-on part is going in the direction you want. The illusion wouldn't work if she weren't a silhouette; you can only make it work because you can pick a body part--her ponytail, her toes, whatever--and decide "OK, it's going in front of her head/ankle this time".
When I first looked, however, I saw her as going clockwise.
hmmm, i wonder if it means anything that i *cannot* make her go
counter-clockwise? maybe it's her shadow, or something else, but i can
ONLY see her going clockwise. although does it count if you see her going
counter-clockwise if looking from the floor up?
Try covering up all of the body except her bottom foot. Then, when her
toes are pointing right, concentrate very hard on the idea that when they
merge with her ankle they're going behind it. Once you've got that firmly
fixed, you can uncover the rest of her. The first few times I tried it she
snapped back to clockwise as soon as I uncovered her, but after a while I
managed to make it stick.