Tai chi is a discipline that offers highly flexible and adaptive objectives, from patients recovering from major illness (like cancer, stroke or heart attack) to athletes taking part in mixed martial art ring combat. Yet, in all cases, there need to be a balance between yin (陰：feminine) and yang (陽：masculine) elements, defined differently according to different objectives. In tai-chi, yin and yang are symbolized in two ways, the black and white fish AND the black and white dots within the fish in the tai-chi symbol. In short, don't forget yin when you're doing yang, and don't forget yang when you're doing Yin. Simple concept!
Take an example. In 24 styles tai-chi nei gung, the first style Golden Turtle is the Yang white dot in the 12 Yin style set; whereas the last style Old Man Cultivating Golden Elixir (老叟燒丹) is the Yin dot in the 12 Yang style set. I did some discussions of the Golden Turtle (or Tortoise) in some previous posts, I shall discuss the latter one briefly below (per a special request!):
In short, the style is divided into three parts. The first two parts are the two directions of front back dantian roll while the third part is meditation along the central channel (中脈), one can choose the whole channel or meditate the heart chakra. Needless to say detail explanation is beyond the scope of the blogging media.
This is an advanced style. Having said that seasoned meditators will likely find it easy (in other words, rather basic) rolling their dantian and doing point or channel meditation even without any external movements, as the 24 styles nei gung required.