Weird. I was having a hard time replicating your problem. Generally you want to rotate your bones on their local axes. Everything works fine rotating on the local y axis of the hand. (R,Y,Y,90). I tried rotating on the World Y axis and I see the issue with the flipping. (R,Y,90)
This should not happen, but I’ve found a solution to fix the issue. In edit mode look at the transforms in the n panel for both the Hand_IK.L and Hand.L bones. Change the z transforms to either .596 for the head and tail or .594 for for both. Right now, they are offset a bit because one is .596 and the other is .594. Change the values so they are equal on both bones and for right and left.
Anyways…not sure why that is happening, but changing these values will fix the flipout.
I would also recommend getting rid of the upper IK constraint like Clockmender’s file shows. It’s extraneous. IK constraints work on multiple bones based on the chain length. Multiple IK constraints are not needed.
Good luck!
Thanks DanPro. This is exactly what I was looking for! Yes, I also like Clockmender’s design and will probably use it for other models. Thanks to you and Clockmender!