In a little more detail: Look at the animation in the article, after the second matrix has been rotated and put on top of the first one. Then flip its top up s

In a little more detail: Look at the animation in the article, after the second matrix has been rotated and put on top of the first one. Then flip its top up so the two matrices are orthogonal. Finally, rotate the whole thing 90 degrees to the left (rotating along the axis that goes from the top of the page to the bottom of the page). Now you can see that the result will be a 2x3 matrix. And the values in each spot will be the sum of the products beneath it.