The spine angle needs to kept as constant as possible during the swing to ensure the body can easily and naturally rotate around it. This starts with having the correct spine angle at setup. You don’t want to lean over too much or be too upright. The perfect angle is between 25-35 degrees, depending on the size of your chest and the length of your arms.
You also need to make sure your shoulders are pushed back and your chest is pushed out to ensure good posture. This is extremely helpful for your swing and also for your back health and longevity. Swinging with a hunched back will lead to many problems in the future.
In the above images you can see a very simple drill using just your arms outstretched to the side. A common mistake here is dropping the left shoulder too much towards the ground so that the left arm points straight down. We want to avoid this and instead train the muscle memory to rotate the spine on the same axis without dipping down. Think of it as a sideways circular rotation as opposed to an up and down dipping movement.
For this we won’t need the Smart Club but will be using it when we combine this concept with the turning of the shoulders. The idea here is just to get the feeling of rotating your spine backwards and forwards while maintaining the angle of the spine. For now just focus on that feeling of keeping your head and the top of your spine fixed and rotating around this fixed point. Don’t worry about the shoulders for now, once you are used to this movement we will focus on adding the other circular movements as well.