I believe I know how this works. And there are many clues to how I arrived at this conclusion. LOL! I sound like sherlock holmes. Here are the clues:
- Look at the channels bar, you see that there is an IK solver connected to the IK control
- There is are two bind controls (same number of shapes on the mesh arm)
- He only rotated the fk's shoulder control
So, my answer is, this is a regular rotate plane IK solver setup with snap enabled. How did he come about having curve controllers? He probably used the command "parent -add -shape". A method to add instances of shapes of any kind unto any form of transform node. In this case, joints and IK handles.
How did I go, kok alex? If you like solving riddles like this, see if you could figure this one out. I posted something up myself. Animating Blendshapes' Per Vertex Weights also available at my site.
The method I described--if I'm right--although much easier to control won't work in an animation. Cuz the minute you set keys, there is a factor that overrides the "floatiness" of the IK.