Kapalbhati is a very very powerful Pranayama which literally means cleaning of the frontal portion of skull. It is one of the shaucha kriyas (cleaning process) which enables the practioner to get rid of his negative emotions e.g. Anger, greed and negative thinking. A lot of energy is spent in doing this Pranayama that may lead to temporary weight loss.
But it is very dangerous to do it excessively.
1. It increases the blood pressure- Paramhamsa Swami Satyananda Saraswati.
2. “Do 10 times in 1 round. If you do it more, a heart attack will come.” Quoted to me by Paramhamsa Swami Niranjananda
3. It results in excessive dryness i.e. both internal and external. - A doctor
4. It reduces sleep, hence causes insomnia in many people.-personal experience
Yoga can be dangerous when practiced under the guidance of an untrained yoga teacher.