The annoucement on my Southwest flight's boarding this week was:

"Folks, if you've got a checkerboard pattern at the bottom of your boarding pass, you must come up to the screening area and present yourself for possible screening. Not all checkerboards will be screened. Those of you without checkerboards are still subject to random screenings. Basically, there must be someone being screened at all times during the boarding process."

I couldn't figure out anything and maybe that's the way wanted it.