Videos in YouTube can be segmented and enriched with extra information using viewers themselves.
When user adds "T=80" to the video hyperlink, the video starts playing from 80 second (YouTube start-end parameters). Then the extra information can be extracted from the context of the page, where the user inserts this link. It is smart.
But usually a user associates extra information with the particular segment of the video (T1; T2), not the segment from time T to the end of the video. What to do, if the segment's end time is not specified in a link ? It can be inferred from the watch time of all users who use given start time T for a particular video. Isn't it nice? Extra information is extracted from a way, in which the users watch the video.