Answer: Prana can be strengthened in many different ways. Each seeker is different and the correct method should be used for each seeker. Usually, the guru decides this. Just as there is no one way to travel…we can travel by boat, plane, car, horse…there is no one way to strengthen prana. Prana can be intensified through any of the five senses: the eyes, ears, nose, tongue, and skin, but pranayam, breathing exercises, is the airplane, the fastest. But not everyone can travel at that speed, so more gradual methods may be best.
All quesions above by Swami Kripalu are from his discourses and informal talks. Edited versions are available in the A Sunrise of Joy: The Lost Darshans of Swami Kripalu, by John Mundhal.