As user types his/her search request in Search Box, several search results are displayed immediately in drop-down list. In addition, the link ‘See all results’ in the bottom of the drop-down list prompts to retrieve the rest of the results. Apparently, the ‘See all results’ has the same effect as just clicking the Search Button near the Search Box. The main purpose of the ‘See all results’ must be to say user: only some of the results are displayed at the moment; you should make a click to get the rest.
I also suppose that the results displayed in the drop-down list are most popular ones (most popular movies in the http://www.imdb.com/ case). I imagine the following algorithm: the site has kind of ‘short list’ of most popular movies. As user types the search request, the ‘short list’ is scanned for a match, and found results are displayed in the drop-down list. Because of the ‘short list’ contains not much items, the search is fast – this is what I see on the site, the drop-down list appears almost instantly after I type a text.