As the sauce reduces, some particles of dried sauce are left on the pan. This dried sauce gets quite hot since it isn't protected by liquid. The sauce contains both proteins and sugars from the meat, wine, and stock. Many of the same wonderful sweet compounds that form when sugar is caramelized are produced. Then, when you stir and splash the sauce back on these dried compounds, many of these are redissolved in the sauce.
Some chefs even use multiple reductions of the same ingredient to get very complex layers of flavor.
Each time you cook the pan dry, you allow the mirepoix and bones and glazy liquid to deepen in flavor. Then each time you add more stock, you dissolve the wonderful brown sweet compounds that were created. These same principles apply to sauces that are an integral part of a dish, as in braises and stews. To make a gumbo, he will start with a little fat and some chopped onions, celery, peppers, and okra in his pan. He adds a cup or so of stock and reduces until the vegetables just start to stick and brown on the bottom.