Just to add my two cents here, Silk Reeling Jin or Chansijin (as I understand it) in the context of Chen Taijiquan has a lot to do with the twisting of the body.
But there is a big difference between twisting and turning.
For example, in the image below, the practitioner on the left swings his knee along with the body. The turning of the body is dragging the knee along with it. Meanwhile, the practitioner on the right is preventing his knees from moving as the upper body is twisting against the anchored leg. This creates a twist in the torso - the ribs and back.
When you rotate the upper segment of a spring clockwise and simultaneously rotate its lower segment in the same direction, you haven't truly twisted the spring. Rather, you have merely revolved it as a whole.
Twisting can only happen when there's opposition in the movement such as anchoring the bottom segment of the spring firmly in place and only rotating the top segment clockwise - you create a genuine twist. It is this oppositional between the two ends that defines the act of twisting.
A lot of Tai Chi (Wushu Performance being the easiest example to reference) do not have twisting. They have turning. And I suspect that genuine twisting would be an ideological clash for a lot of Tai Chi practitioners who worships the word "Song" (to loosen - but some mistakes it to mean 'relax') to such a degree that they hate any kind of tension whatsoever.
Because a genuine twist of a spring creates tension (potential energy). This twist refers back to the 'spirals' that people talk about.
This twist creates a wind-up. And this wind-up is "Internal" in the sense that it's not obvious. If someone pulls back their fist to wind up for a punch, that is very obvious and telegraphed. If someone is creating a twist in their ribs and back while their legs are very stable, that's much harder to see.
And for grappling, it's a bad idea to be swinging the knees left and right while under the influence of the opponent's pressure. That's a reciple to get thrown because the leg foundation is unstable.