Sailing downwind or running is basically when you are sailing in the same direction as the wind is blowing.
As in the diagram below the wind is coming from behind the boat, the sail is eased almost all the way out, meaning that the forward force is maximized.
Downwind Sailing Issues
One problem with downwind sailing is that, since you are traveling with the wind, the wind across the deck, and hence the wind that is being caught by the sail, is less. Another issue is that since all the forces are (almost) in alignment, the boat can tend to become unbalanced, and it can roll over on top of you.
Due to the fact that the sideways force is now minimal relative to the force forwards, sideways slippage will be reduced even more. Some sailors raise the fin as high as possible without letting it interfere with the boom to create less drag through the water, and hence go faster.
Be sure that you check out the short video below that has some helpful tips which discusses the best body positions across all wind ranges for downwind sailing…