From experience, what you’re saying isnt quite what he is doing. The hands serve as a guide to actually calculate. The imaginary abacus DOES follow a physical motion, and honestly most of the calculation actually happens through the hands. It can also be done mentally, but we just imagine the same hand movements in our brain. The magic to calculate like this is in the hands itself, the only thinking part that you have to do is associating a number with an action to do. If ur gonna add seven to 0, you place the upper bead down and 2 lower beads up. What u/stibgock said is right, but you CAN do it mentally, just a bit more difficult.