I'm always amazed at the confusion over the lower body's role in greenside bunker play. Here's a rule of thumb: quiet lower body on the backswing, active lower body on the throughswing.
A lot of amateurs fail to consistently execute greenside bunker shots because they lock the lower body in place and get handsy or armsy with the swing. As a result, they often decelerate through the shot, sometimes leaving the ball in the bunker.
Rotating your lower body towards the target helps propel the ball from the sand and maintains acceleration. Try this swing thought: Turn your belt buckle to the target.
Keeping my lower body and upper body connected throughout is the key to controlling distance from greenside sand. If one or the other is out of sync, you can bet my percentage of sand saves will decrease.
Keep your lower body moving through impact and you'll be a better bunker player.
Belt buckle to the target...