Inviting an individual to state the obvious when they know that your already aware of the answer.
The serve must be relevant to the current conversation/situation otherwise the dive is declared lame. Extra kudos if one can catch out multiple people at the same time with a single serve.
The situation: Friend stood looking at birthday cards in the local supermarket
The 'serve': "...so someone's birthday soon?"
The 'dive': "yes mate it's...."
The punchline: *clap hands together in exaggerated diving pose* "um - DIVE!" *smug grin*