In AmE, how do you use key on in sports? We don't use it in BrE and I know very little about the subject and a not sure how it's used.
Well, in the absence of more detail/context, key on could be/is used in several ways, for example, to mean focus on/target/concentrate on. We will key on the opposing team's lack of skills on defense (any sport). We must key on their receivers (Am. football) because they have outstanding speed.