Stirring a simmering stock incorporates fine particles and fats, clouding the liquid. Avoiding stirring allows these impurities to coagulate and settle, resulting in a clearer stock.
When making stock, you're essentially extracting flavors and nutrients from bones, vegetables, and aromatics. As the stock simmers, proteins and other solids are released into the liquid. Stirring disrupts this process in a few key ways. First, it breaks up any coagulated proteins and fats that are beginning to clump together and rise to the surface or settle at the bottom. These clumps are much easier to remove (by skimming or decanting) if left undisturbed. Second, stirring keeps these fine particles suspended throughout the stock, preventing them from settling. This results in a cloudy appearance. By avoiding stirring, you allow these impurities to naturally separate, leading to a clearer and more visually appealing final product. The gentle simmering action is sufficient to circulate the flavors without the need for manual stirring.
After simmering, let the stock cool slightly before straining. This allows even more sediment to settle at the bottom, further improving clarity when you carefully ladle or decant the clear stock from the top.