What causes the board to spin in as opposed to spin out. The reason l ask this question is because the other day sailing in pretty flat conditions it happen just before l went into a jibe.weird.any ideas why this happen.

When it was happening to me, I was putting my foot too far back on the board. It was helped along by a bit of bad timing with some chop.

I've also had it happen after catching some weed on the fin.....

I think it may be over cooking the unloading of the fin going into the gybe. May be you could actually push the tail upwind instead of down?

I've lost traction going into a bottom turn, when everything is in neutral.
Nothing drastic is happening but I can just feel the loss of traction, if I go into the bottom turn, then I'll spin out, I have to load the fin by going back upwind very briefly, the fin then regains traction and I can pull as many "g"s as I like into the bottom turn.
Since I've been speed sailing, I've learned a bit, and I now think it was caused by having a fairly fat leading edge on the fin. Since I've thinned my leading edges out I haven't had the problem.
How relevant this is to your problem I have no idea, but check the fins leading edge just in case.

Thanks guys for your comments this phenomenon only ever happens when using a weed fin.once in a blue moon next time my board lets go l think l need more mast track pressure or try and snap it back in with the back foot. Honkey donkey.

Had the same problem and found the chord of the fin was too thick towards the tip. Really awkward feeling throwing the board into the gybe and having it decide to go the other way!