U is for the Um Principle
As I mentioned earlier, Tang Soo Do Moo Duk Kwan philosophy is based on Taoism. One of the core components of Taoism is the idea of equal and opposite forces. This creates the balance of life which keeps us humans centered physically, mentally and spiritually.
Um, on the other hand, is the fluid, graceful movement seen in some of the upper forms and more advanced techniques. While still very powerful, these moves are graceful, soft and light. The element associated with it is water- it's a gradual build-up that strips a defense and seeps into the gaps presented by an attacker.
Different forms have different have needs so far as movement goes. The Naihanchi forms, for example, are a good demonstration of the Yang principle- they're hard, punishing movements designed to break bones. Jin Do, Sip Soo and Lo Hai, on the other hand, are good demonstrations of the Um principle- they're meant to be light, fluid and graceful.
Both techniques have a place within the system, neither is better than the other because each is needed for different intents. Equal and opposite, both in their place- creating balance.