In a post on Google Developers blog about the feature, Developer Advocate Thomas Steiner explained the technology behind the feature:
“…with flashing ads, fancy parallax effects, surprising reveal animations, autoplaying videos, etc., the web sometimes can honestly be quite overwhelming… Happily, unlike in real life, there is a solution to that. The CSS media query prefers-reduced-motion lets developers create a variant of a page for users who, well, prefer reduced motion. This can comprise anything from refraining from having autoplaying videos to disabling certain purely decorative effects, to completely redesigning a page for certain users."
Learn more about Chrome 74 beta "Dark Mode".