Scotty,
Good question. The answer isn't completely clear yet.
Without getting too technical, "diffusion" is the process by which several liquids (or gasses or fields or whatnot) mix themselves into a single, fairly homogeneous liquid. When the process occurs across a permeable or semi-permeable barrier it's called "osmosis."
Anyway when you have a salt and/or phosphate differential between meat juices and a brining solution, the fluids "try" to equalize that differential. In that process, as the salt and metallic molecules move from the brining liquid they penetrate the meat. However many of the compounds (solutes) -- including denaturing (as a result of the salt) proteins -- in the meat are too large to pass through whatever barrier(s) hold(s) them in the meat. So the diffusive and/or osmotic pressure builds up and "attempts" to dilute the disparity of solutes by transferring liquid into the meat.
The lack of clarity comes from the lack of any real understanding of the relative importance of osmosis versus simple diffusion. Engineers seem to be enamored with osmosis and insist that most of the action is intracellular. People with actual, strong micro-biology backgrounds are unwilling to commit themselves and tend to advance hypotheses with more taking place between cells than inside them. That is, at least until the heat hits the meat. You can tell from my snide attitude which side I favor, but let's not forget that I'm not qualified to hold an opinion. It's just raw prejudice.
Make sense so far?
Let's backtrack a little. What the hell is a denatured protein? And why should we care? Protein molecules are typically long strands, and in a hunk of muscle, connective tissue, collagen, et alia they're usually wound around one another. When they're exposed to heat the strands shrink and the winding becomes a tight tangle. This is why meat firms when you cook it. If you cook it long enough at a low enough temperature (or cook it to a high enough internal temperature and give it a sufficiently long and gentle rest period) the proteins stretch out and unwind -- that's "denaturing." And it's why barbecued meat can "go past well done and into tender." During the denaturing process the protein knots release moisture whether fat molecules, or plain old "juices" which moisturizes the meat.
So properly cooked barbecue is tender and moist, and we've got to work our asses off to get our techniques "proper." Brining is just another tool in our baterie de cuisine.
I wish I was better at explaining this stuff, but I'm not a science teacher I'm a lawyer. Sorry if I made it more confusing, but that's my gift.
:twisted:
How does that happen? 
