I completely agree with DM--'flying open' is often simply a buzz phrase for 'lack of control on the glove side'.
As his post suggests, the best pitchers control their glove side in a very important way: At footstrike, the glove side begins to swivel into place out in front, somewhere over the stride foot, and stabilizes there as the torso comes forward to meet the glove. There are certainly many superficial stylistic differences between pitchers, but stabilizing the glove and bringing the torso forward to meet the glove are common features of most great deliveries.
Credit where it's due: I first learned of this from Tom House's motion analysis research work. DM's independent confirmation of these ideas shows a great deal of insight on his part, IMO.