It'* doesn't switch directions at all. There are visual cues (including patterns of motion) purposely planted that contradict eachother as to which way it'* spinning. Your brain subconciously accepts one direction of spin based on which set of cues it'* following. There really are no if ands or buts about it. Like "is that a shadow or a reflection of the foot at the bottom?" and such.
That said... I'm not gonna take the test. I've done this type of test lots of times. It always switches around what it says about me.