It's all about physics. Weight slows you down going up hill, it doesn't impact you once you're up to speed on the flats and it makes you go faster downhill. Gearing can provide more downhill speed if you can continue to pedal at higher speeds. Downhill skills can help too but will have more influence on descents with curves and switchbacks where proper timing and body position are more critical. This post is about climbing but it does have diagrams that show the impact of weight in different situations: Secrets to Cycling Faster Up Hills