it's mostly a strength issue, but also a little knowledge issue too.

having taught youth pitching 11-12yr olds for the last two years,  i found that most of them were easily able two get into a torqued position at a stand still, it was during an actual pitch that they couldn't do it.  the flexability wasn't an issue as much as knowledge of how to get into that position during the pitch.  i could tell them to land closed all day long, but it seldom worked.  my son was one of the few that could actually look pretty good mechanically…but he wasn't doing it right either.  once my son was taught the correct way to load and push quickly with his drive leg, it was fairly easy.  most other kids that land correctly actually look pretty slow, indicating a lack of strength in the drive leg.  

you simply have to have enough drive leg power to extend and push your hips open before the shoulders follow.

Brent is a good example of power in action.  the man can squat like a jillion pounds and he also runs a 60 yard dash in 7 seconds!  even though Brent is now in his thirties (poor old man. lol) he can still smoke a fastball past you.

in my opinion; drive leg power is everything.  you simply have to move really fast.  that's it.