The hanging back swing flaw happens when the player fails to shift their weight onto their lead leg during the downswing. This swing flaw really prevents power generation from the core and hips.
When a player demonstrates this characteristic they end up compensating by placing the ball further back in their stance to ensure they can make contact with the ball. It mainly leads to inconsistency in ball striking as well as poor distance.
The tie between the body and this swing flaw can be seen in poor balance/stability in the lead leg, poor mobility or lack of motion in the lead leg (hip or ankle), or even an injury that is preventing the player from loading the lead leg because of fear of soreness or pain. A lack of power and strength in the lower body can also cause a player to have more of an upper body golf swing, which is very inefficient.
Like Lance Gill said above, this exercise is a great way to learn to increase strength in the hip rotator muscles while at the same time allowing the hip joint to rotate both internally (post position) and externally (backswing position).
The above exercise is also a great way to apply a little resistance into loading of the lead leg. There is also a tendancy for the body to fire stabilizing muscles as the band is trying to pull you in an opposite directions of your intended weight shift, which in this case is onto the lead leg. The firing of these stabilizing muscles as well as other larger muscles helps the body to ingrain the movement from a nervous system standpoint. What that means is that your body sends a signal up to your brain and as you are doing the exercise (sending the signal) your body is becoming more efficient at that ideal movement.