I'm not astute enough to foil my own fins, and I love the convience of Futures. Greg Loher and Rich Sanders recently said, when you're on rail, fins 'crab'... thicker foils keep the flow around the fin better attached during this crabbing...better flow attachment, less drag, easier speed.
In a conversation with Eric Roush he talked of "feeding" the Vector force based Futures fins.
It occured to me that when one is trimming high along a wall, you're "on rail" too. That's how we ride, virtually 100% behind the boat.
So, thicker foils, but also, I think this a channel that directs water flow to the low pressure side of the fin.
It's also a GREAT way to cause your glasser to hate you