The MEKO mask creates a characteristic scrambled block pattern in the image, moving all the 16x16 blocks available. Once scrambled, these blocks also have their colors inverted, much like the Negative mask.
Because the mask works with 16x16 blocks and the selection tool usually allows resolution of 8x8, some portions of the region on the right and bottom may be unaffected. They will not be moved, nor will they have their colors inverted.
There are two buttons for MEKO: MEKO+ and MEKO-. Each is the reverse process of the other; MEKO+ undoes MEKO- and vice versa. Using either mask twice in a row will not cause a return to normal, rather repeated applications of the mask continue to generate completely new layouts in a way that can continue for an extremely long time.
As a demonstration, here is an example image with various levels of MEKO+ transformation applied.