The slot is the space between the mainsail and the jib. Interaction between the main and the jib produces a slot effect that greatly contributes to sail performance, particularly upwind. The jib enhances performance of the main by slowing the airflow and increasing the air pressure, thereby reducing the tendency of the main to stall. The main enhances performance of the jib by reducing air pressure on its lee side, thereby increasing airflow, and by increasing the angle of attack at its luff. (The Venturi effect plays no role in this interaction.) It is mainly because of the slot effect that mainsail and jib together produce more power than the sum total of both working alone.
The goal, in trimming for slot effect, is to have the same curve in the jib leech as in the main leech, and make the slot as small as possible, and yet still prevent the jib from backwinding the main. The slot should be small to maximize the velocity of air passing in through the large space between the forestay and the mast and then out through the much smaller space between the jib leech and the main leech. If the slot is too closed, however, the jib will backwind the main, causing it to luff. Hardening the mainsheet, which is the usual response to a luffing main, will only make this worse. It is necessary, instead, to ease the jib sheet (or move the jib car forward) and thereby open up the slot.
of the main caused by closing the slot is most severe on a reach. When reaching, therefore, it is best to trim the jib first, then ease the main until it luffs with a true luff, while not yet backwinded, and then harden it slightly, until the main leech and jib leech have identical curves, but not so much that the main is backwinded by the jib.