DeathMetal, would you mind sharing with us as you learn?  I can do most rigging, but JCMs just confound me.  I'd like to understand how they work, not just how to type one out and I can't, for the life of me, seem to grasp the whole slave/master concept. 

I recently had a project where the JCMs were literally doubling up - acting TWICE on the same JCM - and I ended up trashbinning the project because it was just an unmitigated mess.

I'd like to understand how/why/when that mess came from, though.