And now that I think about it, trying to do this on a regular basis sounds like a pretty good way to boost one’s ability to hold a whole drawing in one’s head and “trace” it onto the paper, rather than doing a rough. I would not be surprised if this ability started as a game he played with himself when he was sitting in the back of his least favorite high school classes, doodling away.
(I am an artist; I can’t do this particular trick but I have definitely had moments when I was totally into a drawing and it felt like I was tracing it onto the paper from my mental image of it, with the hand of a dead artist I’d been studying guiding mine. I was also pretty high at the time; there’s some strains of weed that artists favor because it can give us a boost to this sort of ability.)
My suspicion is that this artist had the opposite -- the ability to maintain a detailed image in his mind's eye as he traced it.
Check out Kim Jung Gi. He's basically doing that in real time, with impossible speed, accuracy and level of detail. He's making a living via performing live shows of his skills.
Not any longer, sadly..
Similarly, Mozart copied down Allegri's Miserere after hearing it just once in 1770. That's a sufficiently amazing achievement that it's still talked about, a quarter of a millennium later!