It's not true that humans only use a fraction of their brain. Using 100% of your brain all at once would just be what it looks like to have a very short seizure resulting in an almost instant death. You don't need to use the "swimming parts of your brain" while you're walking. A muscle can't be both contracted and relaxed. You cant be lifting your arm while also lowering it. If both networks were simultaneously active, you would be having a convulsive seizure.