Comment by pkaler

As another poster alluded to, a practitioner of NLP would call this a "pattern interrupt".

  1) Build up an argument almost to a crescendo  
  2) Pattern interrupt ("it turns out")  
  3) New argument

A practitioner of NLP would say that this weakens the original argument because it puts an interruption between the stimulus and the response.

There is a shock to the system followed by the new argument that you would like the listener to adhere to. The shock makes the listener more suggestible.

That's the theory, anyway. The debate however is very controversial.