Amazing artwork. Neat. Must be difficult to draw so many frames.

Wikipedia does a great esplanation of animation;

The word hentai is sometimes used to apply to cartoon characters that are not anime characters. Hentai is the anime version of pornography. There is on screen sex, often very graphic and very imaginative. Since hentai is anime, the performers are not bound by physical laws. Makers of hentai often use this in very creative ways. Some of the acts performed in hentai would be physically impossible. Some of the performers are often impossible themselves. Fairies, monsters, and even new and strange creatures are stars of hentai. Monsters and demons with tentacles are seen so often that tentacle porn is its own sub-category of hentai.