Golf instructors that just say "stay in box" or "just keep your butt back" is like telling a crack addict...."just stop it...ok just stop taking crack!! :)) Since this has been the major reason I have not had more rounds under par, I have vowed to study way hips move and last year made a huge break through and can give you an easy tip to help everyone practice how to look like Sean O’Hair in the mirror!!! (Prettiest swing in my opinion next to Tiger)
OK Simplicity: (this is hard for me) The X factor (degree shoulders have turned past hips) easily clutters the way your spine should move through the swing and will frustrate you thinking the reason you can’t keep your butt back through the swing or look like Anthony Kim or tiger woods’, is because you aren’t as fit and flexible as Charles Atlas, so you work on core exercises etc and still doesn’t help! It’s because no one has taught you the correct way to move your spine throughout swing, not your hips and shoulders!! So when you work on Posture eliminate shoulder and hip flex to de-clutter!
First go get a copy of golf digest or tiger woods’ how to play golf or another photo stream that shows a notable good tour pro in at least a 6-12 positions in the swing so you can see the way body moves and butt stays back! Take two clubs and hold one club on chest and one on hips. Now copy each position of pro but keep the x factor (variance between hips and shoulders) as close to 0 as possible in other words closest to parallel as possible throughout the whole swing! You still should be able to come very close to looking like the tour pro in a straight line or with a slight reverse c in your finish! And voila you haven’t needed any flexibility or muscle flex to do it!!
Side note: if you watched Tiger and Y.E Yang in PGA Championship you could see this noticeably, both have near perfect posture but Y.E. has min torque compared to Tiger!
Now once you got that down and practiced, add some torque and x factor and enjoy more and more thumping power!!
Please let me know if this helps go try it now! :)