flight of the navigator, Phineas and ferb, Heroes, or any number of science fiction novels.

This "idea" about "using all your brain" has been around a long, long time.

There *might* be *some* truth to it, but it's much more likely that all it would do would be to make processing more slow, but possibly more extensive. When our brains are developing, there's actually a period where the connections are dying off and being pared down into functional, efficient networks (one possible reason babies get better at using their bodies as they grow).

more lay-man: http://www.wired.com/2014/07/everything-you-need-to-know-about-the-10-brain-myth-explained-in-60-seconds/

More scientific: http://www.oecd.org/edu/ceri/31709587.pdf

The way I try to simplify it is:
Imagine that God (Nature, evolution, etc. etc.) wanted to give us all bodies that could adapt and change. So, for the brain he made it like a computer with 100,000 hard drives. Now these hard drives can connect to each other in many different ways, some of which are more efficient than others. As you learn and grow, your brain is switching around the cables on these hard drives, forming new connections, pulling cables from less-used drives, etc. etc.

In that way, our brains can adapt pretty well, so long as we start that learning early. Maybe that's why it's harder to learn new concepts when we're older. So, in the end, you could just "reconnect" all the hard drives, but more likely than not, you are probably just bogging down the system with an inefficient tangle that will make processing time slower.

Now there is always a chance that you could add some extra RAM or Processors, etc. etc. But then you're not just talking about "activating 100% of the brain" you're talking about "activating 100% of the brain responsible for ________."

Personally (and This is 100% my own thoughts, not confirmed doctrine as far as I know), I think that the way we use our minds after this life will be like adding processors and efficient executables.