Chang San Feng was a Taoist sage who lived in the 13th century. Around this time, the Shaolin monks had developed strong practices to maintain good health and defense. One day he witnessed a bird and a snake embroiled in combat. The bird was direct and attacking with its firm beak. The Snake was soft and yielding, waiting for an opportunity to strike. Neither creature won the battle as each was evenly matched in its abilities.
From observing the nature of this conflict, Chang San Feng realized that it is not always the strong that win. Sometimes softness can win. So the idea of Tai Chi was born. When using Tai Chi as a martial art, it is good to have both attacking and defensive abilities: Yang and Yin.
In any event in life, especially a potential conflict, it may appropriate to be assertive, forward, creative; or following, listening, yielding may generate a better outcome. Our ability to have a Yang and Yin response to events is our ability to be open minded, flexible and free.
In Tai Chi we can practice changing from a Yin to a Yang state. So we could start moving slowly (yin) then speed up (yang), Think towards the Tan Tien (yin) or focus on clear, spacious awareness of the area around us (yang). There are many skills to learn and each can be regarded as more Yin or Yang in nature. Identifying which is Yin or Yang is always relative.
Eventually we can combine one Yin and one Yang skill at the same time. Sinking the front of the body (yin) while raising the back (yang) is a classic. We can focus more on one skill and glance at the other, this helps stay focused. So focus on sinking the front while having a regard for its opposite of raising the back. This is a very good method as it develops a deep harmony of energies in the body and mind.