This is an interesting trick to solving multiplication problems. It involves multiplying 2 double-digit numbers with hand drawn, crisscrossing lines.
Say you have a multiplication problem of 13 x 21, you would draw 1 line by itself, then 3 lines, side-by-side, all parallel to each other, to simulate the number 13. Then, you would draw 2 lines perpendicular to the lines already drawn, crossing overtop of them, to create 90-degree angles at the intersections. Then, draw 1 line to the right, and parallel to the 2 lines, still crossing over the lines from "13". Then you count the grouped intersects to come up with the exact solution.
Once you watch the visual demonstration in the video, you'll get it, and you'll use it time and time again for tough multiplying without a calculator.