A phrase used during late fall and nearing the winter solstice by mountain fly anglers as they fish a north/south facing stream . Then, the setting sun is very low in the southern horizon. As the sun gets below/behind the mountains/ridges, the air temperature plummets.
The angler then follows the sun, upriver before it sets; using the warmth provided. Then the angler yells to their fishing partner; Let's make a "run for da sun!"