I saw a link to this ESP test and on a whim checked it out. Try the test a few times to see how accurate it is. Then, see if you can figure it out. It had me stumped at first, but in a few minutes I figured out how it works. Once you think you’ve figured it out, post your answer here as a comment. If no one gets it (or if no one even replies, which is much more likely given the extremely limited readership of this blog) after a few days, then I’ll post the answer.
Hint: You can read through the postings on his site on people’s theories and explanations on how it works. However, all of the explanations given by the people there are wrong. In fact, it’s probably best not to read them until you have figured out the trick yourself.