Well...I don't believe this.
First, it's too easy to 'turn' the spin direction of the dancer or to have the spin keep changing unintentionally. Second, and this contradicts what the article says - my friend Gianni who's a graphic designer saw the dancer spinning anti-clockwise at first. According to the article then he must be a left brain person which means he uses more of the left side of his brain, while I believe he's actually a right brain person because he is a visual and artistic person - afaik.
Although at first I saw it clockwise, but since I read here -where found it- and on the source article that we can see it clockwise and anti-clockwise so I tried it, and yes I can see either way.
I found the easiest way to make the dancer spin as I wish, either clockwise or anti-clockwise.
I just close my eyes or shift my eyes from the dancer and imagine her spinning clockwise if I want to see her spinning clockwise, or anti-clockwise if I want her spinning that way.
As simple as that, no need concentration at all, just a split second imagination.