That's actually a myth. You use all of your brain. Well, I do, anyway. I won't speak for you.
What if this myth was applied to other organs of your body? Could you produce 10 times as much insulin if you concentrated on your pancreas? No.
You could probably limit yourself to using only 10% of your bladder, but you'd have to take frequent breaks and it would be awkward to explain your experiment to other people.