The gist of the matter is, opening one's ball-and-socket joints at shoulders and hips/pelvis (松肩，松胯) represents much of tai-chi Nei Gong. Without that, as one master of Chen-style tai-chi wrote: "It becomes just academic!" (I accidentally dropped by this sifu's website some time ago, but have forgotten his name and the name of his website. I only remember that he was teaching the stuff in UK then).
But opening our ball-and-socket joints is no easy task. In tai-chi's concept of song (松), it also includes evenly strengthening those muscles that are responsible for putting the joints in place, plus allowing maximum movement - with power and speed! Again, the above saying is an objective only. Without proper training, "talking tai-chi" is just like that: "talking tai-chi", or "pure academic"!
Back to the subject of this post. In 24 styles Nei Gong (again for easy reference, one can consult Wang DiXian's book disclosing the physical mechanics), Yang-style number eight "White ape's striking palms 白猿推掌" serves as a foundation style for opening one's ball-and-socket joints. The way to practice is first doing the style with mindful attention in opening one shoulder, then the other shoulder, then one side of the hip/pelvis, then the other side of the hip/pelvis etc...and slip in somewhere with one side's shoulder together with hip/pelvis of the same side.
As per my previous advice, mindfully opening the joints while seeking for maximum resistance is needed (the benefit of seeking maximum resistance is to relax, lengthen and strengthen the connecting muscles). And, in the beginning, maximum opening of the joint should play first fiddler when compared with seeking form-perfection of the style itself!
Enjoy your hard work (who said it would be easy!)