One makes a single random comment in someone elses conversation, which ends up killing it and making it awkward for everyone else. This is similar to a ninja swiftly killing a target then exiting.
Guy 1: Hey, do you see that really hot chick over there?
Guy 2: Hell yea, Id hit that all night!
Conversation Ninja: I screwed her and gave her herpes *disappears*
Guy 1: Oh... Im gonna go now.