(a) an overtly hysterical manner; or
(b) a more subtle, understated fashion, complete with crazy eyes.
The moment is defined less by behaviour and more by the nature of the rant. This is usually a situation that makes everyone else in the room uncomfortable and causes a considerable period of silence. The person can sometimes hit upon the truth in their rant, but mostly this is overlooked due to their bizarre and often inappropriate behaviour.
Named for the many such moments involving the character Jenny Schecter from The L Word.