Grumman HU-16A outer wing slots.
Each wing contains a slot from the tip float out to the wing tip. At high angles of attack, these slots help to keep the airflow attached to the outer portions of the wing so that the inboard portion of the wing will stall first. The benefit of this arrangement is that the ailerons should remain effective up to stall.
The slot is located such that at high angles of attack, high pressure air will be driven into the front of the slot, as shown by the arrows. The cross sectional area of the slot decreases as the air passes through it, additionally accelerating the flow through the slot.