For grammarians, it’s a dependent clause + colon + just about anything, incorporating any and all elements of the other four colons, yet differing crucially in that its pre-colon segment is always a dependent clause.
(Yikes.)
For everyone else: its usefulness lies in that it lifts you up and into a sentence you never thought you’d be reading by giving you a compact little nugget of information prior to the colon and leaving you on the hook for whatever comes thereafter, often rambling on until the reader has exhausted his/her theoretical lung capacity and can continue to read no longer.
(Breathe.)
See how fast that goes? The jumper colon is a paragraphical Red Bull, a rocket-launch of a punctuator, the Usain Bolt of literature. It’s punchy as hell. To believers of short first sentences–Hemingway?–it couldn’t get any better. To believers of long-winded sentences that leave you gasping and slightly confused–Faulkner?–it also couldn’t get any better. By itself this colon is neither a period nor a non-period… or rather it is a period and it is also a non-period. You choose.
via themillions.com