When you trim the sheet on landyachts (which I've sailed) and iceboats (which I haven't, much), you're really working the twist at the head more than sheeting the boom in and out. The boom mostly moves vertically, and the yachts rarely have travelers. If the boom is ever sheeted out horizontally, it's because your going very slowly - like at the start. Once the yacht gets "hooked up" it's sheeted in tight all around the course. You'll ease a bit rounding the windward mark, then back in to accelerate, etc.

So I suspect the DN masts work with the leech tension to enhance the twist control, either at the head or in the belly of the sail.



The other reason for it may be to get a downward force component from the lower part of the rig. In high winds, we loosen the stays on landyachts to let the rig tilt to leeward by 15 degrees or more. This imparts a downward component that helps keep the yacht from hiking and improves traction. The DN mast is doing this automatically - the harder the rig is loaded, the more inclined the lower portion is. Offwind, the apparent wind is less, and the rig would stand up, removing the tilt when you don't need it.

With an inclined rig, the farther the rig is located to windward, the more effective the inclination is, because the downforce has a longer lever arm. It's not practical to move the mast step to windward (although it's been tried on boats), but by putting the inclination down low, the DN mast does keep it to windward as much as possible. The upper part of the rig, which would not contribute anything to reducing the heeling moment, since it's right above the leeward runner, stands straight up for best aerodynamic efficiency: