An old term or not???? Close the gap is something to make your drag lower. When the sail comes closer to the board "air" can't flow between both sailsides. These tip flow can cause a lot of drag "theoretically"

Now I was thinking do I close the gap???? I often or almost all the time do. I remember last wednesday I was sailing on the amstelmeer in my short with bare feet (first time this year), and the bottom of the sail scratched my feet/toes several times each run constantly!! With shoes or surfboots you actually don't feel the sail that much

But I know some sails I have owned never came that far. These sails weren't slow, but I am faster now...

I do have some rules for myself & close the gap. On low speeds the fin will spinout with close the gap, this means end of the run. On higher speed I do know close the gap can result in a big time acceleration, when you know the fin can hold that pressure. If the fin is to small you won't accelerate.