The low wind = soft fin, high wind = stiffer wind makes a lot of sense for me with a full on slalom gear.
Not sure if these fins are still in production.
Not sure i agree with much of this....
Go as soft as possible but as stiff as necessary. Softer fins give more lift at the expense of high end control. By using a softer fin you will have more foiling potential but the fins will be more tricky when sailing overpowered(the fin will kick the board out of the water at a certain point). When you use a board at its lower end wind range(for example a medium 70cm slalom board with 8.6M) you can benefit form a soft fin that generates enough lift to not make the boars sticky. For the upper wind range of a board(for example a medium 70cm slalom board with 7M) with a stiffer fin you will gain top end control and it will help to settle the board down.
For me soft fins are good at dumping the power in the gusts - much like a sail rigged with with an open leech.
Soft fins are not good upwind - again,they dump the power too much.
The Techtronic Falcon and Vector EX are super soft fins and are pretty awesome in overpowering conditions / downwind crazy angles.
For bigger boards like a 70cm wide - again a stiffer fin will surely get you upwind better and will have more lateral resistance to better resist the power of using a bigger sail.
Sailors who keep the majority of their weight on their backfoot - will likely prefer and need stiffer fins.
Go as soft as possible but as stiff as necessary. Softer fins give more lift at the expense of high end control. By using a softer fin you will have more foiling potential but the fins will be more tricky when sailing overpowered(the fin will kick the board out of the water at a certain point). When you use a board at its lower end wind range(for example a medium 70cm slalom board with 8.6M) you can benefit form a soft fin that generates enough lift to not make the boars sticky. For the upper wind range of a board(for example a medium 70cm slalom board with 7M) with a stiffer fin you will gain top end control and it will help to settle the board down.
I think the write up from Buscafins above, sums up the basic pretty well in a few lines - regarding slalom fins.
Lots of sailors think the softer fins gives more control, (I used to think this myself) - but in nearly all scenarios it is the opposite.
You want as much lift as possible (soft) , while still be in control (stiff). Go as soft as possible but as stiff as necessary (This is a pretty good line)
If your board rails up to much in the gusts (like suddenly kicks you out of the water) , then try a stiffer fin.
If your board sits to flat in the water and you can't get it up on the tail / or 'falling' down on the windward rail in lulls, then try a softer fin.
For flat water you can go slightly softer fins to maximise the lift it gives. (as you are more in control on flat water)
For choppy / or ocean water the stiffer fins gives you more control
Rake, outline and fin placement in the fin base of course also have a big impact , together with rig trim. but the above is a good starting point
Peter - for your Vector EX fin, it has a lot more to do with the more curved back outline, that gives you a lot of control and great downwind ability. I know the fin is slightly soft when you bend it, - but again more to do with the outline.
For the upwind ability I don't find a softer / or stiffer fin better. More to do with the overall trim. But yes a stiffer fin is easier to 'stand onto' and push upwind for most sailors, - where the softer can require a bit more finesse / trim, as you can easier 'over power' it.
Jesper
In my experience, a softer fin will allow the board to perform in more of a loose fashion. A stiffer fin seems to add more control and grip through the water but the rider may experience more spinout potential at the same time.
I have recently upgraded to an ELK 71S (soft) fin on my Formula board. This fin creates an incredible amount of lift. So much so, that when it gets overpowered the board lifts out of the water. Which is not so fast! However, when you get it right, it feels just like the board is on foils.
The above description has reinforced my need for a stiffer high wind fin.
I did borrow a 32cm carbon Z fin from Steve Stratfold. Used it on my 87 wood Sonic with a 5.5m in 25 knots and could not sail for being massively overpowered due to the lift 
I have a old 41 cm carbon Leading Edge fin that is super wide and has a super thick leading edge - which makes it off the charts stiff.
Can sail upwind at insane angles on this fin And it gets me going so much quicker than the newer fins.
Downside - across the wind its rather slow. Downwind its seriously slow.
People who sail / trim their board more on their front foot are able to user smaller or softer fins - Peter Dans uses a G10 36cm Vector with his RRD 114 and 7.8m. With that setup I'd go sideways.
Keep all your weight on your back foot and use a bigger or stiffer fin is my way of sailing.
I tried a 34cm Techtronic Falcon with my 87 and 5.5m in 20-30 knots wind and whilst it rocked off the wind,on a reach it was slow. Upwind it was terrible.
Put my 31 cm super wide and stiff G10 Volt back in and was back in heaven!
There are two kinds of stiffness. Bending stiffness ie sideways and torsional ie twist. Lots of bending flex lets the fin take on a curved shape so someof its lift force acts vertically. This kind of bending flex helps the board fly. Twisting flex alows the tip to twist which reduces its lift, this can give some automatic depowering or if too twist flexy can give not enough lift for upwind. A common way to get twist flex is to sweep the tip back so as the fin bends sideways it also twists. A big advantage of moulded fins over machined fins is that the laminate can be designed to have specific bend and torsional stiffness for an arbitrary outline shape and thickness. Eg formula fins often have lots of +-45 laminate so they are very torsionally stiff so they dont twist, but they are very flexible in bending so they bend sideways into a curved shape and make vertical lift like a j hydrofoil. Getting both stiffnesses right is key to high performance and control.
How fast do you have to be going before the foiling effect of a flexible fin comes into play?
Depends on the fin. Could be even before you get planning. For formula boardds definitely a big effect.
I have never understood wanting "twist" in a fin. To my thinking if a fin twists at the tip, but the main body of the foil stays straight, wouldnt the flow of water would be disturbed and cause cavitation/spinout ?
But I know little about hydrodynamics and such.
Can someone explain this further
Lol. Just saw the topic in the gps section. Will have a read.
Some interesting observations in this topic, the above mentioned write up was written with our construction (molded fins) in mind. As written molded fins give the designer the options to independently modify the stiffness (lengthwise and torsional) and influence the coupling between bending and torsion by having an unbalanced layup. This combined with the geometric properties of the fin determine the fin behavior.
The F1 falcon(good fins in certain conditions) is an excellent example where a balanced lay up(G10) combined white allot of swept back(=geometric twist) provide a fins that will de-power under increasing load.
As a fins bends the bended shape will add an upwards force component, hence softer fins will provide more lift than stiffer fins that will maintain their shape more under load.
Ps we still make fins ;) and we just have a new modified slalom design out for 016! Cheers!
Does soft or stiff fins bend same under the board of a rider of 95kg and under the board of a rider of 75kg?
May be the flex of a Soft fin under a 95kg rider could be same than the flex of an extra soft fin under a 75kg rider?
We usually say lighter rider need stiffer fins for control but if they don't have enought power to bend the fin!?
What would be the solution?
