The long tack can change two ways: either by your boat crossing the center of the course, or the wind shifting from one side of the mark to the other. Suppose, for example, you are going upwind on the right side of the course on starboard, which is the long tack. Eventually, that will take you to the center, and as you cross downwind of the mark, it will change from the long to short tack, even though you have not tacked. Suppose, in that same scenario of being on starboard on the long tack, this time you get a header. If the header is enough so that the wind shifts from right of the mark to left of the mark, then suddenly you are on the short tack even though you have neither tacked nor gone very far.