Pranayama is a method of harnessing energy. Inner Vital Forces;
"Prana means breath and Ayama means control.
By Pranayama is meant the control of Prana and the vital forces of the body.
Pranayama begins with the regulation of breath and ends in establishing full and perfect control over the life-currents or inner vital forces.
In other words, Pranayama is the perfect control of the life-currents through regulation of breath.
Breath like electricity is gross Prana.
By establishing control over the gross Prana, you can easily gain control over the subtle Prana inside.
The process by which such control is established is called Pranayama." - Swami Sivananda.