Dantian inner roll (丹田内轉) is an important technique in tai-chi, both for health reason and for martial art application. It is an advanced technique with the prerequisite of being able to do Chi-focused-at-Dantian (氣沉丹田), and the training of the latter will require a good practice of Zhan Zhuang (站樁) as a prerequisite. That is the reason why Zhan Zhuang is called the foundation of all internal martial arts.
What is the relation between this Dantian roll with the practice of creating Golden elixir in Neidan (内丹, or Taoist yoga, Taoist meditation) practice? The seed of the Golden elixir (Little elixir: 小葯) is created through microcosmic circulation (小周天), and shall be cultivated to produce a Golden elixir (Big elixir: 大葯) at one's Dantian through meditation. This Golden elixir is being called "Rolling inner Dantian" in tai-chi's Dantian inner roll (丹田内轉). It is not something "physically" there as an "additional internal organ". And it is not "just psychological". It is a concentrated high-energy chi-mass that a practitioner can FEEL (its presence and its movement) and, hopefully, CONTROL. And it is being used for different purpose in different practices. Let's me explain below.
For tai-chi, this inner roll of one's Dantian forms the center of all tai-chi movements, firstly a power connecting center of all muscles (to facilitate muscles-as-one 肌肉如一) and secondly an explosive power transmitter for martial art objectives (like Bruce Lee's inch punch). For health purpose, able to roll one's Dantian can greatly facilitate chi-flow throughout the body during form practice. The next step is: Learning different practices that aim at applying the skill of Dantian inner roll to achieve different objectives stated above.
For Neidan, creating the Golden Elixir is an important intermediate step towards higher level of spirituality. The next step is: the Golden Elixir is to go through the spinal cord once more, bringing with it one's body and mind en-route. It is called "Harvest the Big (Golden) Elixir to pass through three gates of hindrance" (過三関採大葯).
Needless to say, the above techniques and steps, both in tai-chi and Neidan, are advanced practices for the truly devoted practitioners only. I shall explain the individual processes of the two in due course here. Stay tuned, my dear readers.