This is absolute genius! It makes perfect sense and yet, I had NO idea. Onesies can be pulled UP over the head or DOWN over the shoulders and bum. Obviously this is to prevent getting messes all over or having to do a special roll maneuver to undress the baby after explosive poos. I came across this post on Babycenter and had a slight AH-HA! moment while laying bed last night listening to the snow fall. (aaahhhh, peaceful).
Here's the blog if you're interested: http://blogs.babycenter.com/mom_stories/11162011-am-i-really-the-only-one-who-didnt-know/.