This has be floating around my mind for a few days now, and ive looked at all the discs I own (around 80-ish) and I cannot figure out what the difference is between shock absorbing mechanism discs, and ones that aren't. They look identical to me, bar obvious differences such as colour, brand, etc.   What I mean is the plastic housing itself. What makes the shock absorbing ones different? Or is it some strange marketing thing?