Question on Fly and hands..

Hey all- I'm a relatively new swimmer and am still getting used to the fly (I know, I know- some will say they're 40 years in and still getting used to it ;) ) I recently had a minor breakthrough but want to refine it some more. For reference, I'm going to call the arms forward position 0, and the arms fully back 100, to explain what stage I'm roughly talking about. When I first started, I would keep my hands flat against the water all the way from 0 to about 80- i.e., trying to get as much resistance as I could to loft myself back up. This was absolutely killing me, endurance-wise. My breakthrough came recently when I realized only to push against the water when I needed it, i.e., just at 0, and then again at around 60 when I needed to loft myself up. Put another way, I only "turned on" hand power when my feet were ready for the second kick. This made me much quicker and easier-going. So my hands would slap down at 0, then turn fairly loose till about 60 when my feet were ready for the up-moving kick, then they would help out and push me up. My question is, after watching a video, I didn't realize that moving your hands into a knife-like position was actually desired at some stages of some strokes. By knife I mean sideways so you are cutting, not pushing, the water. I had always thought you want to be pushing against the water always to push yourself forward the most, but it's now becoming apparent you only want to "turn on" your hands at certain points. Is this true? And most importantly, how does it apply to fly? When are you "pushing" and when are you "knifing"? And is this really what the "s" motion I keep hearing about is getting at? I know this is beginner stuff, but hey, so am I at this point ;) Thanks!