Whenever the opposing player moves into square 13 he sets the trigger on what we call the "left jumping off point" (LJP). The LJP is a place where a sacrifice of 1 piece will result in the taking of 2 or more pieces and will often end in jumping in for a king. The LJP is sprung moving the piece at 21 or 22 to square 17. In the current example the LJP is used to exchange 1 piece for 3 and jump in for a king. See if you can see how?
