There are going to be a number of things in a full golf swing that could be highlighted as been one of the more important factors in its efficiency. A simple comparison of a number of great swingers will show they have certain things in common and one of those is definitely a dominant rotary force. This rotational force is what helps to make great players develop such high club head speeds at impact and subsequently long distance. However the more sway in a swing the less chance you will have of taping into and using this rotary force in a consistent way. The less lateral (sway/slide) or front to back (loss of posture/early extension) movement that occurs in the golf swing, the more rotational force a player can develop. Weight shift in a swing when it is done properly becomes part of the rotary movement and should not be a lateral sway of the body.
Although this movement can be enhanced and practiced on the range it is also something that can be improved off the course with some simple exercises. Here are two that will help the flexibility range and separation mobility.
Lie on the ground and lift your right leg up so it is bent about 90 degrees at the knee. Stretch the right arm out along the ground so you can feel the ground with your arm and shoulder. Hold the knee with left hand and gently pull the knee down to your left side. Try to keep the right shoulder on the ground whilst trying to reach the ground with your knee. Switch legs and repeat on the opposite side. Try to do this every day, it will only take 1 minute but will definitely help your rotation.
Hold a club in front of you and hook your foot behind your knee. Keeping your upper torso as still as you can twist the hips one way then the other a good number of times and with a good amount of speed. Change legs and do the same again trying to keep balanced whilst rotating from the hip area. The idea here is to progress this exercise so you don’t use a club in front of you but have your arms folded across your chest this will help improve balance whilst rotating your hips.