Kapala Bhati
Kapala Bhati is a basic Pranayama technique (detailed instructions are found in the chapter on "Pranayama").
Kapala in Sanskrit means skull and Bhati means polishing or purification. Using fast breathing, the flow of Prana through the Ajna Chakra and the frontal lobe of the brain is regulated, hence the name Kapala Bhati. This technique cultivates mental alertness and is therefore recommended as a practice before meditation. Kapala Bhati clears the mind from thoughts and creates a calm and aware awareness. On the physical level it helps removing mucus from the body, cleanse the lungs, enhances carbon dioxide and oxygen exchange while, at the same time, also improve the flow of blood to the brain.
Yogic breathing techniques, or Pranayama, are practices that cleanse wastes from the body. Kapala Bhati is mentioned as being one of the Kriya in addition to being a basic Pranayama technique.
When the body is pure and clean, the effect of the Asanas on different bodily systems is greatly enhanced, and the practice becomes much more efficient as it allows a more harmonious flow of Prana, the vital energy.
There is no need to practice the Shat Kriyas every day. These are only necessary when external, unwanted materials accumulate in the body.
At the place where Hatha Yoga ends, Raja Yoga begins. Raja Yoga includes one of the most important books entitled, "The Yoga Sutras of Patanjali Maharishi". In it, Saucha is mentioned (internal and external cleansing).
Kriyas are part of the external cleansing. Internal cleansing is expressed by cleaning from anger, jealousy, selfishness, frustration, fear and so on. Internal cleansing has immense importance due to the profound affect that negative feelings have over the many systems of the human body including the nervous system and brain.
The practice of Yoga aids nature in its natural process by discharging wastes from the body that cannot be normally expelled alone.