HAP=harmony, KI=body and mind coordination, DO=the way. Uem / Yang, or the concept of balance through "full circle wholeness" (mental, physical, spiritual) is the cornerstone of Hapkido philosophy. Using your opponents "energy" against them has no religious meaning. It's simply physics, you will see as you read further. Spirit refers to the person (who you are). Mind refers to emotion and thought (choices). Body refers to the physical body.
The key principles of the Hapkido philosophy
Yoo (Water - Liquid/Solid/Gas)
Won (Geometry/Circle)
Hwa (Physics/non-resistance)
Seang-Ni-Hak (Physiology) Meom (Body)
Shin (Mind) Shim-Ni-Hak (Psychology)
The 'Ki'
The Ki (as in Hap-'Ki'-Do) refers to the coordination between the mind and the body. Although the word refers to this, it literally translates to spirit, or energy. It originates from the Chinese word 'Qi' which is pronounced in China as 'Chi'. For reference here, it must be noted that many Korean words are developed from Chinese.
Core practising philosophies
A well performed technique in this style of martial art should have the following Hapkido philosophies in mind:
Unbalancing motion
The attackers motion should become unbalanced
Redirecting
The force and direction of energy of the attacker should be redirected
Decreasing the radius of the circle
This point is a fundamental point taught in Hapkido. A number of the techniques are based on this philosophy. It means to increase the potential power and effectiveness of a move by concentrating the movements closer to a pivot point. This type of motion in Hapkido is best seen in the inward spiral of a wrist throw or rotation and pivot used in the foot movement of most throwing techniques. Imagine a whirlpool of water, the closer to its pivot, the more power and energy it appears to have, by dragging all the energy from its surroundings.
Lowering the center of gravity
Generally, although not in all Hapkido techniques, the idea of lowering your belt to a level beneath your opponents will reduce your center of gravity and increase the chances of obtaining a better leverage to carry out a techniques (depending on the situation). This is most effective in throws and grappling.