This is getting fun...mainly because we are able to share experiences, ideas, and knowledge, even if we don't completely agree, don't agree at all, or find different ways to get there.
Root - I think when you state "hurdling" you are talking about the front leg swinging open to get the hips square to the target, correct? If so, I am not sure I agree that it does not matter how you square the hips to HP, I believe you want a "swinging gate", and not a "revolving door". I have always said, the lower half mechanics are very similar in pitching as it is to hitting. You want a firm / somewhat closed off front side, to rotate the back side around. If you rotate the entire piece, sure you ensure complete "squaring" to the target, however I don't think you create the "elastic energy" between the top / and lower half....at least not the same as if your "firing" mechanism is the back hip being the dominant rotating component.
Each sequence influences the next, and I don't think they are completely static & segmented movements, they influence, and hopefully make the next sequence more powerful. Otherwise, why not have the old "Mike Marshal" delivery where you step towards the plate square from the "get go" without ever going sideways. I hear your statement on the slingshot, however I will ask if this is the case in pitching, and hitting, why not just hold the bat in the "power slot" then swing, and not have any gained momentum from let's say a "tip & rip" style of hitter (i.e. Gary Sheffield)?
Jolietboy - not a fan of Brent P...., and I would not construe BOF or any others as argumentative....I think your topic has yielded a great discussion.
I realize I'm in the minority on "hurdling." Here's why. I used to spend a lot of time "fixing" that problem with kids, including my own son. I was pretty proud of myself for doing so. Then, reviewing video I noticed my son had gone back to "hurdling." Being way too detailed, I have pretty good velocity records from bullpens for my students. A closer look showed me absolutely no difference from before and after fixing the "problem. In fact, I noticed more than once (especially with mine). Velocity actually decreased a tic after the fix. So, I had to ask myself, why does everyone teach it? Asking around got me either no answer or answers about momentum and the kinetic chain. A little more study and viewing video leads me to believe that any momentum built in late hip rotation is an unnecessary component. The momentum built is focused in the wrong direction. I can't see any proof that it adds anything to velocity and may actually decrease from stability. I now focus on hip/shoulder separation without worrying too much as to how it is attained. For some pitchers, it may well be a mechanism that aids in getting max separation, but I no longer think it's a teach. Again, I know I'm in the vast minority. I've just tried to stop teaching anything unless I can explain why I teach it.