Basically, you count the number of men in the image, wait a few seconds for the top two pieces to switch, and count again. The left top side of the image goes up and slides over to the right, while the right side of the image slides to the left.
In this image, there are 12 people hanging around. Go ahead and count them
But, when you counted the ones in the final image (after switching), you only counted 13, right? The question is, where does the extra man come from?
The “extra” man is created by having 1 man NOT have a new top and 1 man NOT have a new bottom when the shift is made.
In other words, if the shift was a direct match up, each bottom would have a corresponding top. So 12 bottoms would match up with 12 tops, creating 12 men. But when you don’t match up a top with a bottom, you have 11 complete men and 2 “half” men. And in this case, the missing halves are so tiny, our eyes don’t see anything as missing in either of these 2 “half” men. Therefore, it effectively creates the illusion of 2 “whole” men rather than the true “half” men. So when you add up the original 11 men with these 2 illusionary whole men, you get 13 men total.