Do we begin speeding-up at the beginning of the stroke or only at the end?
When does the SUAS begin and when did the rest of the casting stroke end (leading up to it)?
to me the answer is the same for both questions, that is the accelaration is continous as the rod is loaded more. there is only the push and the stop. to me the stop is the determining factor in my loop control. if i bow the tip slowly, as you alluded to, it will open up more. i use this to be able to push the line speed tight loop theory but then open it up at the end to get slack for dry fly presentations. or also moved horizontaly to reach a mend in the air. or on good days both at the same time.
this brings up the point of fishing vs casting. my fishing casting looks ugly at best, but i could care less, all i want is that dry to land where i want it with enough slack to reach the riser. i rarely cast single handed for practice these days because it makes me think about casting instead of being able to do it like walking down the sidewalk. the more i think and analize the worse it gets. it was the same thing when i played goalie in hockey. in that situation it is called "fighting the puck" . i see beginner casters do this every day, "fighting the cast". at a certain point it must become automatic, doing it without thinking. great topic.