You Know What Meaning

(idiomatic) A phrase used to get someone's attention before announcing something.

Example: 2001 May 10, "Blake's Attorney on Bakley's Murder", CNN Live (transcript)
  Well, you know what, he's got a cloud over him. You have to live with who you are and make your peace with your maker about what you really are.
2009 March 23, Alex Hawgood, “The Three-Ring Circus”[1], New York Times:
  Well, I guess I’ve got to say, I love the Madonna one that we did — the Drowned world tour. Actually, you know what? The first Blonde Ambition tour I loved. The “Vogue” thing — that was a memorable moment in my lifetime.