To begin and understand what energy manipulation is, you must first understand that anyone who performs magic, no matter the amount, uses energy manipulation for their spells. How do they use energy manipulation for their spells? Well, magic is the use of energy combined with intent and focus. When you cast a spell, you generally direct where you want your energy to go depending on the spell being cast.
Energy manipulation, or EM, is the direct manipulation of energy; the psi ball, for example, is EM exercise. A fact that you may not know: when you practice various EM exercises, you work on controlling your own energy.
Why is control important?
Whenever you cast spells for any reason, you use energy. Having better control means less energy is wasted into the spell. Let's say you pour so much energy into your spell, but it ends up either failing or doesn't work correctly. When you learn to have better control over your energy, you should be able to cast spells without exhausting yourself mentally, physically, nor emotionally. You can also exhaust yourself from over-focusing. Not only does control over energy help you properly cast spells, it also makes you feel less exhausted from casting in general.
What are the requirements for learning energy manipulation? Well, the biggest thing you need is to focus your mind on your energy. Meditation should be able to help with that. The next requirement is determination and the ability to draw onto either your own energy or the energy around you.
How to draw upon energy
To draw upon your energy, you really need to visualize your energy in the central area (stomach, near the heart, etc). Visualize a pull on that energy and pull it in the direction and or path you want to. Once the energy is gathered, you may feel a strong sensation in that area. You want to draw the energy to your fingertips and then out to be released. You can try to gather energy from there and create things such as a psi ball for control. You can go beyond the psi ball exercise to help work on control a bit more.
Types of energy
You may have heard that different types of energy exists. Does that make them separate individual forms of energy? If so, then it's a lie. The "types" of energy are nothing more than using energy with an imposed thought. For example, if you use "healing energy", you are generally thinking about healing someone. Or perhaps when you think about harming someone, you generally would think of using "dark energy". Now converting the "type" of energy from one kind another is pretty simple. You need to have a "type" and bring it to a neutral state (energy without thought, emotion, idea, etc.) Then, you impose your new thought or emotion on that energy, converting it to another "type".