The harmony of twist and flex is a very important criterion for the fin function. Both twist and flex should react in a progressive manner.
Rather soft fins turn out to be good-natured if speed is not too high and in manoeuvre. Across the wind and upwind they tend to lose the track and to suffer spin-out.
Rather hard fins are offering a good performance across the wind and on upwind courses but they tend to deliver problems of control, of spin-out and quite often they are not the fastest.
Fins with very hard flex and very soft twist are bad on upwind tracks, they do not respond.
Fins with very soft flex and very hard twist are also bad on upwind tracks, they tend to suffer spin-out, they do not respond.
A good fin is an “intelligent” fin, that means its flex and twist adapt and respond to the changing stress rapidly so there will be no break-down in the flow. A good fin will support the sailor actively on all courses maintaining maximum speed.
Good wave and free-ride fins will support the sailor especially in manoeuvres; slalom fins should permit high speed especially across the wind and on a broad reach and should be neutral in manoeuvres. Whereas, Formula fins of medium hardness and very little twist should gain a huge upwind speed, and they should be controllable and very fast on a broad reach.