"A leading edge slot is a fixed (non-moving) opening behind the wing’s leading edge. The slot does not operate at low angles of attack, like those found in cruise flight. At low angles of attack the airflow just passes over and under the slot.

At progressively higher angles of attack air starts to move through the slot from the higher pressure air below the wing to the lower pressure air on top of the wing. The mixture of the air coming over the leading edge and through the slot has greater momentum and thus sticks to the upper surface of the wing to a higher angle of attack than if the slot were not there"


Makes more sense to me now!
Afterthought, Why would you want them on a pattern plane if it will affect the higher speed performance though?