RE: q for ian diffrence downforce code red and warp 2007
"Downforce" is not my word, but commonly used to describe what I refer to as "dynamic weight", that's to say, a sail(rig) in use, under load imparts a load (downforce...) onto the board greater than it's static (actual component) weight.
As the mastfoot is the pivotal leverage point of the rig, you can see that the forward vector of the sail's "lift" loads on the mast track, itself also part of a lever (or leveraged point) in the boards longitudinal trim. Jim would define it better.
Different sails - and even different tunings of the same sail - influence that loading, which in turn (can be used to) influence or optimise the trim of the board. Varying the mast foot or strap location, you're adjusting the leverage "length" of rig (or rider/straps), in turn trimming (balancing?) the balance beam (board).
It's all a subtlety, but so is fin flex- and what a deal that turns out to be.
Cheers ~ Ian