Earlier quoted context omitted.
This was quite a popular technique to achieve fancy video effects on low powered hardware. A good example is the various parallax and scrolling effects in the intro cinematic to Link’s Awakening on the Game Boy/Color, achieved by changing various hardware scroll registers in the horizontal blanking interval (time between one line being finished and the other starting). The main limitation is that you can only do hori…
I seem to recall the article being about Apple II hardware, but it might have been Atari 2600. They are of similar generations. edit: Yep, you are right - the article is titled "Racing the Beam" https://www.wired.com/2009/03/racing-the-beam/
In theory, you could, but on the Atari 2600, during screen drawing, the CPU cycle budget for each line was 76 cycles (see https://cdn.hackaday.io/files/1646277043401568/Atari_2600_Pr...). Rounding up, that’s 40 instructions, and if the current line is different from the previous one, you’d have to update memory from that same budget.
Because of that, reading of user input and updating of player/object positions typically/always was done during the vertical blank interval.