For those of you who would like to further your knowledge of yoga, I would like to recommend the following book: Moola Bandha (The Master Key) by Swami Buddhananda, published by the Bihar School of Yoga.
The book is suitable for any yoga student who would like to understand more about the effects of the bandhas in a yoga practice. It is a fascintating read, describing how to use the bandhas in order to activate the arousal of Kundalini, the primal energy within.
The three bandhas are the Jalandhara or throat lock, Uddiyana or abdomen lock and the Moola, perineum or root lock. There is a fourth bandha, which is a combination of the three, called maha bandha.
Very little has been written about the ancient practice of bandhas. This book has a theoretic and practical approach to the use of bandhas, as well as covering pranic and psychotherapeutic aspects of using these locks. Practical advice is given in order to help you to find and use them effectively.