Question about Hentai (in manga) censorship

I noticed that in some Hentai mangas, there is a black line which covers a part of the penis between the glans and the body.
My question is: Why is that part being covered?

Because of silly japanese censorship laws, instead of turning a penis into a black bar or pixels they cheat by just drawing a black bar over the part you mentioned since then it still counts as censorship but still shows plenty.