There are 2 reasons that bendy mast work and make you go very fast. just for refrence,iceboat masts bend a hell of a lot more then this picture in a big puff. Between the mast base and the hounds 12' up the mast, the mast can be bent at least 2 feet out of column

1. The lower part of the mast bend allows the sail to impart downfource on the leeward runner which increases grip of the blade on the ice.
2. The dn mast sail combo is very dynamic. When a gust hits,instead of the boat hiking (flying a windward runner), the mast bends below the hounds,and the top of the mast twists and bends off to leeward. The top of the sail blades off. What this means is that the when tuned correctly, the DN is effortless to sail. Up wind you sheet in all the way once you are up to speed and the mast does all the rest. If the boat is hard to sail and hikes alot in puffs you tuned it wrong.

As far as why the leeward shround is loose, that should be easy to see. The mastbends, lowering the distance from the base to the hounds a few inches and the plank bends up a few inches, this alows the mast to lean a few degrees to leeward and the leeward shroud in loose.

What this all translates into is a boat that can be competitively sailed by women that weigh 110 and grandparents. Last year I finaly got fast enough to beat all of the 80 year olds! A good mast does all the work, you just hang on and steer.

Cheers,
Eric