J.M. Hollerbach constructing a mechanical arm that can produce handwriting;
" Coupled oscillations in horizontal and vertical directions produce letter forms, and when superimposed on a rightward constant velocity horizontal sweep result in spatially separated letters. Modulation of the vertical oscillation is responsible for control of letter height, either through altering the frequency or altering the acceleration amplitude. Modulation of the horizontal oscillation is responsible for control of corner shape through altering phase or amplitude."
the effect of phase shift on the velocity space ellipse.
I suspect that in a lot of asemic writings the 'rightward constant velocity horizontal sweep' is replaced by a variable sweep with a variable velocity.
Here's a machine with a vertical oscillation and a chaotic horizontal kick that causes a short phase shift, superimposed on a rightward constant velocity square sweep.