When recovering from a concussion you want to keep your activity level within capacity at all times. If you do so, then your brain will heal. As your brain heals, the capacity line will rise. As that line rises, you will be able to sustain more and more activity without aggravating your head.

But if you exceed capacity, then that line will drop a whole lot faster than it will come back up. And no matter how low that line goes, you have to stay below it.