Suppose you have 7 couples and you want to do a 3 couple dance in a 4 couple set. You could split the couples up into a 3 couple set and a 4 couple set, and, in the 3 couple set, have the 1s move to the bottom after they do the dance once. Or you could put the two sets together, and have the 3 couple set borrow a couple from the 4 couple set when it needs one.
The basic idea is that you line up in a 7 couple longways formation and divide the set into two parts. There's a three couple set at the top, and a four couple set at the bottom. The three couple set at the top is almost normal. The 1s dance their part twice, but then they stop. They don't move to the bottom, they stay in 3rd place. Then the original 2s dance twice and stop, and so on.
The bottom four couple set's job is a bit more complicated (though it will be familiar to anyone used to a triple minor). The first time through everything is normal. The second time through their original 2s will be out at the top. Except here they aren't out. They are "borrowed" by the 3 couple set above them to dance as 3s. They finish being 3s, and suddenly they are now 1s in the four couple set.
Any 3 couple dance in a 4 couple set can be danced in a 7 couple set if desired.
And, of course I have an animation:
It's possible to add another 3 couple set on top of the first (giving 10 couples total) then the middle 3 couple set will both borrow and lend as needed. I've never danced in a 10 (or 13 or 16...) couple set, but it is technically possible.
And if you continue logically, with a lot of couples (3*n) you could do a sort of round-the-room (or sicilian) circle where everyone dances twice as 1s and borrows or lends to surrounding minorsets as needed. I've never seen anyone else even mention this...