This is useful for checking whether your display uses sequential scan refresh or global refresh. Sequential scan refresh will show a tilting effect on a vertical moving line. Sequential refresh is usually better for lower input lag since the display can refresh in sync with video cable transmission (DVI, VGA, HDMI, DP) but such displays may have more visible scan-skew artifact or jello scrolling effect at lower refresh rates (e.g. 50Hz or 60Hz). Line may tilt as little as one degree. It is easier to see scan skewing at 60 Hz than 120 Hz. It is easier to notice skewing when viewing at a further viewing distance of over 2 screen widths away.
If you are testing a smartphone (e.g. OnePlus 5) or an iPad tablet screen, try out both screen rotations. Sideways-scanning is vulnerable to jello effect during vertical scrolling at 60 Hertz. Computer monitors rotated in portrait mode is also affected by jelly scrolling. Higher refresh rates (120Hz) and/or vertical scan reduce jelly scrolling artifacts. If you are testing a gaming monitor, try both with and without your Blur Reduction mode (e.g. LightBoost, ULMB, DyAc, etc). Faster "Speed" shows jelly effect better.
Technologies with scan-out skewing: CRT, OLED, LCD (when flicker-free/non-strobe)
Technologies without scan-out skewing: Plasma, DLP, LightBoost, ULMB
Technologies with sawtooth artifact: Some billboard displays