If you were heading towards a jump and you are engaged with the jump (as in it's obvious you're heading for it and counting down the strides) then for whatever reason, you need to circle, your horse will have to go over the line you already made.
It's quite literal. Pretend you are in a sand arena, and heading for a jump. You can see the tracks of the horse in the sand, right? So if you need to circle, there's a good chance you'll have to go back over the tracks. It can also happen if you get lost and keep moving, you may accidentally cross over your tracks again.
It doesn't count when you are not actively moving towards a jump. For example, the line you took to jump 3 is not relevant if you have to cross over that line to get to jump 8.