Even working passively, the part of the brain is still working alongside the parts of your brain dedicated to active tasks. Altogether, a vast majority of the brain is functioning at the same time. Some actions may be unecessary at a given time so that part of the brain remains relatively dormant, but what I was trying to say was that we don't use only 10% of the brain, but a vast majority of it at most times.
Granted, we cannot control every action of or brain, but there wouldn't be a use for doing so. Regulating liver processes isn't on my to-do list. If the brain is working passively, there is a good reason for it.
Typing this up required a lot of actions to be done at the same time. Memory, dexterity, spell-checking, thinking, motor skills, and, since I'm eating, chewing my dinner at the same time, while doing this, I'm also planning further into the night (when I should begin my HW, etc.). All of these actions require much more than just 10% of the brain.