On the "hard-soft" scale of martial arts, Hapkido stands somewhere in the middle, employing "soft" techniques similar to Jujutsu and Aikido as well as "hard" techniques reminiscent of Taekwondo and Tang Soo Do. Even the "hard" techniques, though, emphasize circular rather than linear movements.
Hapkido techniques follow three principles:
Non-resistance or harmony (화 = Hwa)
Circle principle (원 = Won)
The Water/Flexible principle (유 = Yu)
This is simply the act of remaining relaxed and not directly opposing an opponent's strength. For example, if an opponent were to push against a Hapkido practitioner’s chest, rather than resist and push back, the Hapkido practitioner would avoid a direct confrontation by moving in the same direction as the push and utilizing the opponent's forward momentum to throw him.
Circular Principle (Won)
This is a way to gain momentum for executing the techniques in a natural and free-flowing manner. If an opponent attacks in linear motion, as in a punch or knife thrust, the Hapkido practitioner would redirect the opponent's force by leading the attack in a circular pattern, thereby adding the attacker's power to his own. Once he has redirected the power, the Hapkido practitioner can execute any of a variety of techniques to incapacitate his attacker.
Water Principle (Yu)
This is analogous to the term Jū used in the names of Japanese arts such as Judo and Jujutsu, and can be thought of as the soft, adaptable strength of water. Hapkido is "soft" in that it does not rely on physical force alone, much like water is soft to touch. It is adaptable in that a Hapkido master will attempt to deflect an opponent's strike, in a way that is similar to free-flowing water being divided around a stone only to return and envelop it.