I have not seen this before, but one problem that comes up when looking at this diagram, is that if you find the total area of the large triange:
1/2(bh)= 1/2(13X5)=32.5 square units.
When the tiangle is broken up into sub units the sum of the areas = 32 square units on both triangle neglecting the "hole".
If one were to put the two tiangles together to form a rectangle, you would have an area:
Area=bh = 13X5=65 square units.
The sum of the sub-units in the triangles would equal 64 square units and your missing square would total up to 65 square units.
