In case anyone was wondering, I think I found the answer. It’s what would be called a moderate stop. Not too abrupt or too slight either.
On this webpage:
Judging the Yorkshire Terrier by Dr
There’s the interpretation that an abrupt stop goes hand in hand with a more rounded or dome shape to the skull, which of course is incorrect for the breed. This is something that kinda surprises me cause I always thought that a flattish type of skull could be had even with an abrupt stop. I swear I’ve seen yorkies with an abrupt stop with a squarish looking head that appears kinda flattish on the skull. But apparently not. Maybe it’s just a certain camera angle or something that’s given me that impression in those cases I think I saw that.
Anyways, it seems the answer is the muzzle is at a 90 degree angle to just the head in general, not any particular part of the head. And also that the stop should be moderate so that way the skull won’t be too rounded or dome like in shape. A moderate stop should allow the top of the head (the skull) to still have some flatness to it.