Simon learned this trick from a Numberphile video. He made mental log sticks himself. He first programmed the nets in JavaScript and printed them out, then filled in the grids by hand. I helped him cut the grids out and glue them together. In the video, Simon uses the trick to quickly calculate the sum of four four-digit numbers and explains how he did it. If anyone wants to make these, here is a picture of the nets/grids:
Geeks out of the box 