I have never understood that...OK, so you have some leeway while sailing, why wouldn't you want BOTH rudders working for you when they are in the water? Foils work more efficiently with reduced loads and if you can share the leeward load between both rudders... I think someone somewhere noticed that with the toe-in, when they fly a hull and come back down, the windward rudder doesn't leave as much of a splash/trail as it re-enters the water hence it "must be faster"...then they won a regatta somewhere.