Similarly to other videos like the 'headless papyrus' one this video was the result of dwelling a little on the prospect of if sans had reacted to the second hit during UNDERTALE's 'Genocide' run, after all, he'd already been prepared for the first hit in the actual game.
Back in undertale's heyday in fact there were already a couple of videos exploring the same idea, all with different outcomes and I thought I'd throw my hat in the ring as well!