I think of a "jab" as a short, self-contained comment that has some kind of impact. Something you can slip in while your opponent is talking, or as a side-comment in a longer monologue.
This may have two positive effects (for the purposes of "winning" the debate):
On the other hand, you risk lowering the audience's esteem of you.
2009? - Mos Def vs. Christopher Hitchens on nuclear weapons\
Hitchens: Mr. Definitely, can I call you that? (laughter from audience)