ExhaustedSpark's link didn't open for me. Here is an excerpt from the article on virtualweberbullet.com:
The Mechanics Of Letting Meat Rest
In the book How to Cook Meat, authors Chris Schlesinger and John Willoughby state that as meat proteins are heated during cooking, they coagulate and squeeze out some of the moisture inside their coiled structures and in the spaces between the individual molecules. The heat drives this liquid toward the center of the meat. In the book CookWise, author Shirley O. Corriher reveals a bit more detail: as meat proteins cook, they begin to shrink. Up to 120°F, the proteins shrink in diameter only and there is little moisture loss, but above 120°F the proteins also begin to shrink in length, which really puts the squeeze on moisture. By 170°F, most of the moisture will be squeezed out of a lean piece of meat.
As meat rests, this process is partially reversed. The moisture that is driven toward the center of the meat is redistributed as the protein molecules relax and are able to reabsorb some moisture. As a result, less juice runs out of the meat when you cut into it. Willoughby claims that if you cut into meat right away, almost twice as much liquid is lost than if you let it rest before carving.
Good luck and good smoking.