The ‘slot’ is the gap formed between Headsail and Mainsail. The best way to sum it up is to say
that if you have a sail behind a mast, you can improve the airflow over the lee
side by careful adjustment of an overlapping headsail. The slot so formed will
speed up the air flow over the lee side of the main and reduce the turbulence
caused by the mast. But the slot is not a propulsive device of itself: if you
were to make a single sail equal to the combined area of main and jib, and if
you were to hank it on a forestay so that it was free of the mast’s turbulence, its
propulsive effect would be greater than that of the two sails and their slot.