Bend test is a big part of what I use to tell if the ribs are done. Grab the slab around the middle and let it flop over............the saying is "if it points at your feet its ready to eat"...........ribs that still need a while will be a lot stiffer. Very unscientific, but it seems to be pretty reliable. Of course if they just bust in half you went to far.......here is another pic of a different slab from the same batch that started to tear.......just a bit over done but still good eating IMO.
In all honesty, I don't use it much anymore mostly becuase I have smoked so many damn ribs I can usually tell if they are ready without picking it up. I took the picture to send to my brother-in-law who was not really grasping the concept over the phone. Use it in conjunction with any other method you currently use to tell if your ribs are done.
I believe that when the meat is first put on the collagen fibers tighten up and make the meat stiff...........as the cooking goes on the collagen breaks down (that is the point that you see the temp plateau in briskets and butts) and no longer has the strength to hold the slab out firmly.......but just remember "when it points at your feet it is ready to eat". LOL Hope this helped.