[around the curve of the foil]And why does the water move faster?
For a similar reason that the outside of a record (or CD for you technophiles) moves faster than the centre - it has further to go!
Beware, if fluid (even something with a relatively high surface tension (inter-molecular forces that we won't go into detail on)) is stressed too much the forces will break, releasing trapped air (like boiling), whence comes cavitation. I assume a small amount of this is good, since it provides the "lift". Too much and we get drag.
I guess the question is how far from the fin these effects stretch. Is it even possible for fins that are inches apart to influence one another or are we using the wrong key in the dark?
Does anyone even know whether we are dealing with laminar or turbulent flow in surfing? Some enthusiast could make a few calls about the real (rather than land) speed of a board and a fin in turn and calculate the Reynold's Number. Then we may deduce how far from the fin the effects are likely to extend and find out whether quad fin synergies are just smoke up my skirt (not me, I've already got one PhD on the go, thank you)
