The slats are normaly held in by the airflow but as the A of A increasese the C of P moves forward and the stagnation point ahead of the wing tends to move down and under the leading edge it is this high pressure air under the slat and the drop in pressure above the slat as a result of the C of P moving forward that deploys the slat just at the time when the boundary layer needs re energizing to avoid the airflow seperating from the upper surface of the wing thus increasing the A of A is which flow seperation happens =lower stall speed.