Wait what, you got instructions for yours?
I have just received my first Sanhui doll too, with the Flexi skeleton, and yea it's definitely quite different from the old 2015 hinged skeleton I'm used to. There seems to be a mix of different types of joints throughout the body rather than just ball-joints, which is something you need to get used to since they will behave differently. And don't use too much force by pulling the legs up/down by the foot or bending the back by the head. Instead put a hand on the joint you want to bend, and pull with the other from a position not further away than the next joint. That way you will not put too much stress on welds and other joints. It can be a bit challenging at first until the joints loosen up a bit.
As for how to get a doll into more complex positions, it requires some planning. Learning how the joints work and the easiest/safest techniques to bend them in your environment is vital basics. Also always keep in mind what may not be healthy for the doll, as in how the silicone stretches and folds, as well as compression against delicate areas like gel implants and makeup.
Another good advice is that instead of brainstorming/freestyling positions, get a collection of actual photographs of people in the positions you want to achieve. That way you can focus more on planning than than improvising. You may have a good idea in your head, but most often it lacks the details that makes the pose look natural in the end, and that's what some form of reference material of real people is good for.
The basic steps I personally go by is something like this;
1. Surface, make sure it's soft/stable enough and wont stain the doll, or vice versa considering what foot studs and oil can do to floors and furniture.
2. Basic position and balance. This only concerns joints needed to be positioned for the doll to form and stay in the pose, mostly just the legs and spine, and arms only if they are needed for support (if not just keep them safe and close to the body). It's usually easiest to adjust the limbs when the doll is still laying or sitting, before you carry the doll into position.
3. The details. At this point the doll is where you want it to be, and you now focus on the rest of the joints to get arms, hands, and head into natural positions and angles. Also remember to make use of the twist of the spine, which can often make a huge difference for the end result.
This may sound like a lot to think about, but you'll quickly learn and do it fast and fluently over time.
As for hands. On mine they seem to still be hinged for up/down movement, and rotate by the whole bone of the lower arm. If it's been rotated off it's natural position, you can usually see how the silicone twists to know what way to rotate it back. In these cases always adjust it in one direction at a time.