Apparently you have never fenced professionally or otherwise. The point was while flat on your back decreased mobility makes it more difficult to defend yourself, not the other way around. This isn't samurai sunday mud where a nifty spinning sword is just going to knock all those sharp pointy things from coming even close. While not everything may be logical, nothing I can think of is the complete reverse here. It would be like saying because you are sinking in quicksand and there is less of your body to defend, you should be able to dodge and parry better. Nope.