냐면은

'What it is, is this:' — self-answered question

-냐면은 poses a question and then answers it yourself, holding the listener's attention while you set the point up: 내가 진짜 또 대단한 게 뭐냐면은.. 'and the really impressive part is...' The shapes that dominate here are 뭐냐면은 'what it is, is' and 왜냐면은 'the reason is'. The trailing 은 is a spoken stretch and adds no meaning.

Register: spoken, explanatory