If you read a little further down he states that the .65 ratio
"sits better in the wind" for FAI.
By this i take that to mean that when the heli is hit by wind gusts during precise hovering manouvres it tends to stay nailed
put over one point more
rather than the flybar responding too aggresively (heli moving into the wind direction) or not enough (heli moving away).
I think that's the right way around
It may be the same paddles/flybar setup used for his 3D
respond a little
too agressively to wind gusts at the .85 setting (or the correction is too powerful)
So to get the machine good to go
for both disciplines
without changing the flybar length or paddles, he just changes the ratios,because
at the ratio nearer to 1:1 it corrects (flybar responds )too agressively into the wind gust direction and over compensates (like a gyro with it's gain too high)
So the ratio you think is more stable for FAI .65 is actually less stable but he could be using it for different reasons than you think or with a heavier setup on the flybar that differs to his 3D setup.
When thinking about this stuff it's best to think in terms of how a heli reacts to a deliberate control input vs how it reacts to outside influences
The flybar damps both, or slows down the reaction of the blades to more manageable levels (speeds)
If it were a perfect setup,an FAI flyers dream (or even a 3D flyers dream)
you'd have a heli that had total
neutral instability,it stayed nailed over one spot when the sticks were left alone and it was trimmed neutral cyclics
,moves directly in full proportion to the stick input and start stops moving exactly in time
with the stick input,as fast or as slow as you wanted.And if it got hit on all 4 sides it would stay exactly over one point,even while pirouetting on the spot
in a 30mph wind .But i think it'd make flying heli's too boring/predictable