How can I set an IK limit on a bone's Y axis? I'm trying to rig a leg that has two thigh bones (one to move the leg around the hip and the other to rotate the thigh around the bone's main axis):



The IK constraint on the Shin bone reaches up to ThighB; ThighR has X and Z locks enabled on the bone's IK tab so as not to bend the leg between ThighB and ThighR. This works fine.

I tried to lock the ThighB's Y axis on the bone's IK tab but this has no effect - ThighB still changes it's roll when I move the leg (I also tried using a limit and setting a high stiffness, this also didn't work):



So I have two questions:


What exactly is the "Limit Y" / "Lock Y" option good for, since it doesn't seem to work as I expect it to?
Is there a way to keep the IK Solver from rolling/rotating ThighB around the y axis?


I'm aware that I could set up a second chain that just used a single bone and then copy the x,z rotation to the ThighB and the roll or the y rotation to ThighR, but I'm looking for a simple solution first.
With no pole, locking the Y axis works, and can be seen by locking/unlocking it in pose mode.  However, what exactly it means to lock the Y axis isn't necessarily intuitive; in reality, orientations aren't sets of 3 independent axes.
There is one situation where locking the Y axis (of the first bone in the chain) will do nothing: when you are using a pole target.  In this case, the calculations used to track the pole will replace the limited Y axis rotation of the first (only) bone in the chain.
Note that pole targets aren't necessary for IK; they're an option, and there are alternative controls.
If I wanted to create an IK thigh twist bone with a pole target, here's what I'd do:

The thigh bone of the IK chain is non-deforming.  The deforming thigh twist bone is parented to it.  Note the hierarchy: the deforming (upper, non-twist) thigh bone is not parented to it.  Instead, it's parented to the hip (or whatever the IK thigh's parent is), and then acquires the proper orientation via a damped track constraint, in this case, targeting the head of the thigh twist bone.