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The first time I barbecued a brisket, I took it out of the WSM and sliced it immediately. I was surprised to see a rush of juices run out all over the cutting board! Afterward, I wondered why the meat was so dry?
My experience is an example of why it's important to let meat rest after cooking and before slicing.
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.
America's Test Kitchen Radio did an experiment in which they cooked pork loin roasts at 400°F to an internal temperature of 140°F. Here's how much liquid each roast lost after different resting times:
Just a 10 minute rest resulted in a 60% decrease in lost liquid, and a 40 minute rest resulted in a 90% decrease of lost liquid—and even after 40 minutes, the internal meat temperature was still hot enough to serve.
The benefit of keeping more liquid in the meat is that our perception of tenderness is greatly affected by moisture content. Moist meat is softer and perceived as being more tender than dry meat.
Have you ever noticed that the internal temperature of meat continues to rise after removing it from the cooker, grill or oven? This is called carry-over cooking and is caused by residual heat transferring from the hotter exterior of the meat to the cooler center.
As a general rule, the larger and thicker the cut of meat, and the higher the cooking temperature, the more residual heat will be in the meat, and the more the internal temperature will rise during resting due to carry-over cooking. This means the meat must be removed from the heat at an internal temperature lower than your desired final internal temperature, allowing the residual heat to finish the cooking.
Most well-documented recipes on the Web and in cookbooks will specify the resting time, so follow the author's advice. Otherwise, here are some guidelines for various meats.
Note that thin cuts of meat like ribs, chicken, fish, and thin steaks and chops need almost no time to rest after cooking. They just don't have enough mass to hold a lot of residual heat, so there is little, if any, carry-over cooking. By the time you get them off the grill and onto your plate, they're ready to eat!
For example, when roasting a prime rib at 325°F and targeting a final internal temperature of 135°F, remove the meat from the fire at 120-125°F, cover loosely with foil, and let rest for about 15 minutes. The roast will rise to your final internal temp during the rest.
During the resting period, cover meat loosely with aluminum foil to prevent the surface from cooling off too fast. Make sure the meat is on a cutting board with a "gravy groove" or a rimmed baking dish to catch any juices.
The only exception might be pork butt and brisket, which are often wrapped tightly in foil and stored for several hours in a dry cooler until ready to serve. See Holding, Storing & Reheating Barbecued Meats for details.
If you're looking for some general guidelines for cooking times and internal temperatures for various meats, see Cooking Times & Temperatures.