According to many pros the left side of the body (right-handed golfer) should be firm when you shift your weight forward. However, every time I look at sequence pictures of professional golfers I do not see this.
If you look at only a still picture it is possible to see that the weight has shifted and the left side is firm. However, the picture does not tell you when all of this took place. But yet this is critical to understanding the sequence of actions in the golf swing as well as how and why they occur.
For example, weight shift is needed to establish a firm left side around which the hips and shoulders rotate. It is the weight shift that establishes the firmness because your weight is now on the forward leg and the muscles are tensed to hold this position. You can’t shift weight into a firm left side!
You can prove this to yourself very quickly. Try to firm up you left side without having your weight on the front leg. You will find it impossible to do. If you firm up your left side while still in the basic stance position, you will have to relax the left side in order to allow the weight shift to move forward and then get the left side firm.