I am desperately trying to wrap my brain around some of the cool concepts in DW.

I'm struggling at when the GM 'acts'.  There is no traditional initiative that I've come to rely upon so I'm not really sure when the GM makes moves. 

In the intro of the pre-release, the GM inflicts some damage on one of the characters and I'm not really sure why that happened.  Because the focus is never to declare the move itself, it wasn't clear, from my point of view, why the GM was using a move and against that character. 

Does anybody have any insight here?