When the 14's were moving to t foils (as against T foils) Bieker who pioneered these foils argued that the foils effected the flow at the transom which was like lengthening the waterline. I've never really looked to closely at it but if you could lengthen the "hole" left by planning this might result in a boat the acted like it had a longer waterline, it would definately increase the "hull speed"

What I have seen in practice is that the boats are generating lift with the rudders up wind and as a result the crews are standing well back. This then allows the bow to become finer and therefor faster through the water. Down wind the finer bow isn't the risk it used to be beacuse the foil can be used to keep the bow up. This is why the foils (or the cases) are made adjustible, so you can get lift upwind (and in low wind down wind) and down force in a blow.