Comment by weinzierl

Exactly! The C64 could control where the beam started painting. To move the screen a pixel you just wrote the the x and y offsets to two 8-bit I/O registers. Only after scrolling 7 or 8 pixels you had to copy memory around. I was relatively easy to get this right smoothly and since everything was in sync with the beam it was easy to make tear free.

Shaking effects that did not require memory copy were even easier.