Competence porn is basically, like, someone is really really good at something and it's written in such a way that even if the other characters aren't marveling at it, the author is definitely making a big deal of it to the reader. It's not dramatic tension: the joy comes from seeing the character just kicking epic amounts of ass. In movies/TV it often involves a montage to establish someone's skill.

Bucky is generally considered to be very very good at killing people (I mean, that's canon!), but people don't always play that up in their fics. I am one of these people, because I am bad at writing action sequences. But I love reading fic where his skill is emphasized.