Part of learning to use these fracture pans is learning to position your patients on them correctly. If the patient's anus is on top of this "flap" (the flat part of the pan), yes, any defecation is going to be blocked by this part of the pan and end up smeared all over. It'll only happen to you once because it's not fun to have to clean it up. You have to make sure you position the pan so that when the patient does defecate it is going to end up in the pan itself