This is a great video that explains, very simply, how a wider surface area spreads energy. Of course, this is the concept behind TAK Polycarbonate Training Knives. A wider surface area and light weight allow higher intensity, and therefore more realistic training. A step further gets you to Nok Hard Contact Training Knives where padding is added into the mix.
Jason Thalken, a physicist, has produced a series of very interesting videos explaining the dynamics behind fighting, impact, momentum, etc. I really enjoyed the videos and think they are worth watching. If you are a fighter, you have a basis to understand what he is talking about since you have experienced it. If you are a beginner, his discussions could lead you to being a better fighter.