Before revision: If a boat tends to round up when the helmsman is attempting to sail straight downwind , and you cannot reduce sail area, but the boat is manageable on the other points of sail, then the only solution is to avoid that situation by tacking downwind.
After revision: If a boat tends to round up when the helmsman is attempting to sail straight downwind , and you cannot reduce sail area, but the boat is manageable on the other points of sail, then the only solution is to avoid running down before the wind by instead("running down before the wind") tacking downwind.