They use evidence collected about the user, like typing history on social networks, where they are on the Web, or what terms are trending on the Web to determine what word to select next.
The video can be linked with the electronic dictionary so that a user can select a word in the subtitle during the replay of a video and quickly check its meaning.
This has been used by many researchers worldwide to create BCI-based applications that allow users to spell a word, identify images, select buttons in a virtual environment and more recently, even play in an orchestra or send a Twitter message.
Just highlight the word or passage of text in the browser window that you are interested in and then right-click it and select the Hyperwords menu to see your options.