Resting meat after roasting allows the fibers of the meat to relax. The juices, which expand during roasting, shrink as the meat cools so the joint deflates and the fibers consolidate. The meat is then firmer and easier to carve. If you try to carve it straight from the oven, the slices will be ragged and steam from the hot juices can cause scalding.
Large joints weighing more than 2 kg should be covered in foil and rested for 20-30 minutes. Small roasts need 10-15 minutes and can be rested uncovered.
** Asian Recipes