Correct flap settings depend on what airspeed you are doing (I think!) -

Positive Flap (flaps down) for flying slow in thermals and light lift - and probably for landing phase - increases lift / slow speed efficiency.

Negative Flap (flaps up) for speed gliding between thermals - increases high speed efficiency.

I've only flown one sailplane with flaps and that is how I was taught to use them. I was always admonished for not returning my flaps to neutral (from flaps down) when leaving a thermal and going on glide... and for not setting them to negative (up) when speed gliding - students just can't win!

Phil.

PS - Cool video - more, more, more..!