hatha yoga

I am reading a book titled Yoga and Health.
It's discussing hatha yoga, which is a more relaxed and gentle form of yoga.
The text states that,
"The Hatha Yogi who has reached the highest level of ability has complete and absolute control over his body. He can regulate at will the activity of his heart, his digestive organs and the functioning of every other organ in his body."
Yogis that are 80 or 90 years old seem as if they are 30 or 40 years old.
They live to an old age because they can recharge their bodies with new life energies at will.
Cool.