Often times in anime, I see a serious section accentuated by a gritty, more serious, more old school, animation type. There will be many frames of animation, and the animation will be very fluid. Usually this is done within a light-hearted anime, presumably for contrast with the main part of the show. The example I have is not the best one, but gives you an idea of what I'm talking about.
The opening scene of Golden Boy has a super serious vibe to it, and then goes into the regular style for the anime. This is a screenshot of the scent in the opening I am referring to:
This YouTube video has the opening, and the first 3 seconds are the most "serious style" of the opening. Note that not in all cases are these clips of animation visually dark like this (for example having a black background.) I think this art style is more often present when depicting fantasy anime, but I'm not sure.
Is there a name for this style of animation? Ultimately, I would like to look for anime that use this style of animation throughout, so I am looking for something I can use to search The Internets for (aka a search term).
If I can think of another example, I will edit this question to add it.