The great secret to mastering prana itself is learning to shift the flow of breath at will with the power of attention and without the use of fingers or any other external aids. Learning to shift the nostril flow from the left to the right nostril and back is not an intellectual or analytical process. It only requires you to mentally observe. The very act of observing the breath at the space in front of the left or the right nostril activates that particular nostril. Even if the particular nostril is passive, by paying attention to it, it becomes active.

Shah Grouven, Radhika. Mastering Pranayama: From Breathing Techniques to Kundalini Awakening . first Publishing. Kindle Edition.