Almost every class has a twist or two in the beginning and again in the end. Twists early in the class are done lightly. They help with opening your spine and to develop mental focus. Twists at the end of the class are deeper and restorative, helping to realign the spine and relax your body.
Twists help with flexibility by stretching and lengthening the back, waist and side muscles. Toning the oblique muscles helps to improve posture. Sitting or standing posture is important to the well being of your internal organs and your energy level. Good posture keeps you from collapsing and compressing your back, vertebrae, and internal organs. Compression can cause problems with your entire body.
Twists improve circulation throughout you body. A common theory is that twisting squeezes blood and fluids out of your internal organs (liver, kidneys, spleen, etc) allowing fresh blood to flow into the organs nourishing and lubricating them. This flow of blood and energy (oxygen) revitalizes cells, tissues and nerves. Digestion is improved. Bloating and gas are reduced.
Twists encourage the synovial fluid to lubricate the lumbar vertebra and the hip area. Twisting helps with all the little muscles that support the vertebrae and keeps the whole structure of your spine free and mobile. A flexible spine helps you feel and look younger. Twists can also reduce headaches and improve both eyesight and hearing. This is because you are releasing refreshing your neck and shoulder areas.
Twisting releases tension. Most people really enjoy them because of this big release of toxins and stiffness. They quiet the nervous system. When tension is released and relaxation sets in some biochemicals are stimulated help you become calm and focused. Twists are relaxing and energizing all at the same time.
Here are a couple of sources to read more about twists.