avx > doctorgonzo already explained the reasoning why getting under an overpass isn't good, but as for diving in a ditch, yes, they say that works too. You're basically trying to get as low as possible because a tornado doesn't just totally suck down to every little inch of terrain it can possibly get into. Imagine it having a flat bottom that doesn't bend much....kinda like its surfing on a big invisible surfboard. If it's a quarter mile wide and you jump down into a ditch that's a few feet deep, you're lower than the larger part of the ground that its going over. It won't bend and come down into the ditch to get you because that would require bending that little part of the vortex to go that extra direction. Much the same way you can stand behind a small hill on a normal day and the wind doesn't hit you as much because it goes over the top of the hill and right over your head.
I got confused on the one about what to do if a tornado is going to hit your house. One choice was to lie flat in a bathtub and another was to cover yourself with a mattress. They picked to just lie in the tub, but around here (in tornado alley BTW) they tell us to get in the tub AND put a mattress over you!
I disagreed with the one about punching the shark in the eyes too. I know sharks are really protective of their eyes, but I too have heard punching them in the nose is supposed to work as well, because that's where a bunch of the sensing nerves are located and punching the nose gives them a jolt kinda like hitting your funny bone real hard. I would think finding the nose would be easier too.....I mean, if you're under water, you're gonna have to be looking around for its eyes, whereas the nose is easy to find; its right in front of where you're being bitten!