A 'crab' is when your oar blade doesn’t stay perpendicular to the water and either digs way down into the water or pops out of the water during the pull through. The more common crab is the former kind and it results in the oar getting stuck in the water at the end of the pull through. Since the boat is moving along quickly the result is that the handle of the oar whips past you, if you are lucky, and ends up parallel to the boat. The ones you can see on you tube are the more dramatic type when the handle of the oar catches you in the gut and can actually toss you out of the boat.