Could someone please tell me what difference a flex tip fin makes and when it is best to use one?
A stiffer fin creates a bit more lift and thus is better in lighter winds.
My understanding is a bit limited and is wavesailing oriented but in the waves a flexy tip fin makes for a more forgiving bottom turn as it allows better absorption of bumps in the bottom turn. Thus a lighter rider would prefer a flexy tip, or normal to heavy people in overpowered conditions with a bumpy wave face.
As a heavy guy I use stiff fins for most sailing but at Gnaraloo for example where it is plenty windy and bottom turn control is more important I use a pretty flexy fin
Bit like sail twist I think, the twisting off allows it to dump off some flow and therefore loading on the fin
Sort of 
i thought a stiffer fin was better at higher speeds and a softe fin better at low speeds"
anyways, thought i'd have a crack at this also but i am sure i will get something not quite right. there are some guys on this forum that have lots of experience here.
ok, there are lots of things here, there is twist, flex, torsion and rake "i think".
generally,
a raked back and soft fin like a wave fin will produce more twist
with wave fins a flexi tip is desirable because it allows a lower angle of attack before stall occurs due to the twist, but a soft or twisty fin is "generally" slower.
for slalom fins a soft tip (no where near as soft as a wave fin) will allow some twist to occur at the tip which can produce a faster fin as the tip will "twist" and reduce drag but the flip side is a loss of lowend performance
a stiff tip or "thicker wider" tip slalom fin will usually be a little bit slower but produce better upwind characterisitcs and be faster off the mark as they produce more lift.
that's a basic explanation as chord ratio, flex, twist, aspect ratio etc all also have an effect.
Check out vmg blades web site. It has a good tutorial and whilst it is formula specific the same principles apply
