The idea behind Smart Motion Previews to video results is to help users decide if it is a video they want to go watch, therefore eliminating the need of clicking back button in their browser soon after clicking on a search result. “The Smart Motion Previews give people the equivalent of a movie trailer for video results. When we crawl videos, we create short previews (never more than 30 seconds, made up of a few very short clips) that reflect what our video crawling technology thinks are the most relevant parts.  We look at the audio levels for instance to see a big play in a sports video (based on the applause from a monster dunk, for example),” explained Bing Search blog.