Do you understand how this test works?

I just posted about my trouble figuring out candy crush saga and I think it may be a similar concept to this test that I have been unable to figure out. Despite having superior intelligence in many things, my skills at this certain test are apparently equivlant to somebody with a severe intellectual disability. I've had this test at psychologists office several times and even reading about it I cannot figure it out even though it's said to be very simple. Supposedly people with schizophrenia and other certain neurological disorders are unable to understand how it works. I'm curious how many people understand it. It is a test used to determine someone's executive functioning. (It's called this Wisconsion Card Sorting Test.)

http://www.millisecond.com/download/library/v3/CardSort/CardSortTest.web