The sail is simply a wing operating on its side. i.e it's vertical to the ground instead of being horizontal to the ground. But the laws governing it's behaviour are still the same as any wing.
It is easy to slide out under even quite light winds if you pull the sail in too early before the forward speed builds up. In effect you are either stalling the sail or at least, operating it closer to it's stalling angle of attack.
Then, all you end up with is the sideways force while losing the forward thrust component of the lift generated by the wing (sail).
The most effective angle of attack for a wing is around 15 degrees. This is the angle which you should try to keep the sail to the apparent wind to get the most forward thrust for the minimum amount of sideways force. i.e the minimum amount of force trying to make the board spin out. If you increase this angle by hauling the sail in, you don't immediately lose all forward thrust but the ratio of forward thrust to sideways force rapidly diminishes.
The result of this can be that even though you can feel lots of force on your arms trying to hold the sail in, it is operating more as a parachute and you get little forward thrust, acceleration is slow and the board keeps sliding out.
It is actually quite difficult to sense this optimum angle because the feedback you get is mostly just the force on your arms, so the tendency is to think that by pulling the sail in hard, you must be getting lots more thrust because you can feel more force on your arms. This is NOT the case. Sideways force does not equate to forward thrust.
And it's the forward thrust that you need.
The sideways force is an unwanted result of generating the forward thrust. It is just wasted effort and should be minimised. As the forward speed increases you can progressively pull the sail in because the apparent wind then acts to reduce the angle of attack.
At any speed it is still best to keep the angle of attack to the apparent wind at 15 degrees. You can find this point by continually letting the sail out a few degrees and check if the board slows down. If it doesn't slow down then it means you have got the same forward thrust for a reduced sideways force and consequently, a reduced risk of spin out. I know everybody always says to pull the sail in & close the gap but while this may help drag the last knot out of a breeze that's exactly right for the rig, it will certainly result in more spin outs under anything but ideal conditions for the board/sail/fin combination which you might be using. Hmmm,.. this is an awful lot to say on the subject. I should have just said, try letting the sail out a bit. But anyway, now you know why.
