It's ambiguous. Get used to these optional illusions enough and you can switch the way you see it turning at will. I do it by telling myself that the one part is in the front and then it's in the back when I want to switch.

Edit: if you want a kinda trippy effect (and get good enough at switching by will), try telling yourself the arrow is moving to the front all the time and it will smoothly spin both CW and CCW each cycle.