wouter,
Although I am no engineer, It does not take a genius to figure this out.

The curved track must compensate for the radius at which it turns causing slack in the leech unless it is then again manually sheeted.
As the wind force forces the car to leeward on the straight track, the leech must tighten, there is nowhere for the car to go other than further away from the clew unless manually travelled to open the slot.