Have you ever cooked a steak at home and wondered why it never ends up quite as succulent and tender as it does in a restaurant? Blood runs out over the plate?
When you cook a piece of meat, the muscle fibers that are in closest contact with the heat contract. As fibers contract, all the juices that are nestled between them get squeezed away from the source of the heat. It's just like squeezing a sponge.
When you cook the meat on both sides, all the juices flee from the edges of the meat into the center. If you cut the meat open, edges will look brown and cooked, but the center will appear bloody and raw.
Instead, take the meat out of the pan, place it on a warm plate leave it to stand for a while. Between 10 and 20 minutes is usually about right. As the meat slowly cools down, the muscle fibers that were so tense before start to relax. It's the reverse of the sponge effect.
As the fibers relax, they re-absorb the juices from the centers of the meat, and draw it back towards the edges. The result - cut open the steak now, the whole of the inside will appear evenly pink. The residual heat from the edges will have cooked some more of the center area.
Relaxed meat is more tender and succulent than tense meat, because the juices--and their flavor--have been reabsorbed rather than wasted.