Pablo Dobarro shared this preview of Sculpt Mode - a way to post models without having rig your models. It automatically adds IK bones as needed 😱.
The new Pose Brush with IK also has an alternative mode that controls the roll rotation of the bones. The brush falloff defines how much rotation is propagated through the chain.
The implementation of the brush is much simpler than it seems. For example, it uses the IK solver explained in this Coding Challenge by @shiffman which only has 21 lines of code. I added comments to the patch to make it easier to understand how it works.