It's makes so the frame pacing fits the display refresh rate, making viewing a lot more pleasant and smoother. Eg.: Your Screen is 60hz, but the video is 59.97hz, then there will be a dropped frame. `--video-sync=display-resample` speeds the video up by a tiny percentage to make every display refresh match a new video frame. The correction factor is recalculated every frame and can be viewed by pressing I. This makes way more difference that one would imagine and makes 60hz gameplay look hyper smooth, even though not much changed. Frame pacing really matters. Applies to very closely matching framerates or multiples of (eg. 60hz screen with 29.97hz video), not to bigger differences like 50hz vs 60hz.
Drawback: MPV will be forced to run at screen V-Sync, draining battery faster. If your screen is 60hz, but the video is 29.97hz, MPV will re-draw the screen at 60hz, instead of just 29.97hz, doubling GPU load.