I am using auto-rig-pro on my duck and for some reason, no matter what I try, when I rotate the hand 180 degrees on the x-axis, this happens:
It’s already lo-res (30,758 faces) and I already used heat maps and twist bones.
I checked check-marked these already and still nothing works. What do I do?
Sorry that is an add on and not used in Gamedev courses, I doubt anyone is likely to have or use it. But you never know someone may have it and can help.
For this reason, your best bet might be to try and fix the rig manually with your own new weight-painting, and quite possibly bone roll in this case.
Since it’s an add-on though, you might be able to find a forum or t-shoot guide that discusses it specifically.
Welp, at least I tried asking.
Indeed, good luck! We simply don’t know enough about the add-on to suggest anything more, so I hope you are able to find what you need =)
Difficult to understand!
I am not too expert at this, but where are you trying to rotate the hand from? It has a reasonable result to me depending on the point you rotate it from and what you rotate, which does not mean that it’s the expected result (and no, I don’t know what the markings mean, so my apologies if it is clear from them). I am just acquainted with how things in the body bend, that’s all 
.
I mean I rotate the rig of the hand on the world axis.
When working with bones, there is/was a bone-option to “Keep volume”.
Blender thentries to keep the volume of the transformed mesh.
But to be honest, I do not understand your problem. Turning 180 will do the twist.
I need the arms rotate like a human’s arms.
Your arm is too short to do a nice twist then, use the whole arm to do the rotation, then the twist is less visible. Experiment with manaul weight painting
I’m assuming you’re using IK. There’s a pole target you need to move it around as well to rotate the elbow. If you’re using FK then you have to rotate the upper arm first(usually between 2 to 15 degrees.) then the forearm(5 to 20 degrees) then the hand( 130 to 163). Note: yes I know it doesn’t add up to 180 but human don’t do more than 160. Exception are double jointed and really flexible people.
As @Dwayne already pointed out, if you look at your hand rotating to the maximum extent, you will see that the hand does not rotate all alone. It rotates partially from the wrist, the forearm, the elbow, the upper arm, and even moving the elbow sideways and accommodating the shoulder. And everything linked to the bones moves partially around them, so even if you go for a simple rotation part by part, you’ll need to be generous with the area allowed to move with the weight paint (except those parts that cannot move so much).
