Let's start at the beginning YOU. You're already familiar with the source of magical power. You've felt it when you've run up a flight of staires, watched an effective horror movie, frown angry, or felt submerged in love. This is personal power, the energy that stems from our physical bodies.
When we're in good health, our bodies are capable of producing tremendous amounts of energy, as any athlete who's pushed past seemingly human limits will understand. This energy derives from the earth, for our bodies transform food, sunlight, fresh air, and water into available power. Normally, we use this energy to mantain good health and for such daily activities as exercise, work, sleep, study, thought, and sex, among other things. In magic, we transfer some of this energy to other purposes. Personal power is raised (through tightening the muscles), focused with a goal (through visualization), and released to bring that goal into manifestation. This is a simple process, and isn't much different that focusing your attention toward studying all night, running a marathon, finishing a work of art, or writing a poem. The intent, if not the process, is the same: we direct energy toward completing a project.
In magic, however, we consciously release this energy so that it can speed away to create the changes that we need.
Personal pwer us usually released from the body through the projective hand. This is the hand used for writing. If your ambidextrou, use either hand.