These mannequin drivers are “soft” constraints, meaning that they could be compromised when kinematics is being solved. And if you add a “hard” constraint on top of the soft one - the system will simply ignore the soft one. The problem might come when you change the kinematic solver type to a one that does not handle overdeterminacy. But otherwise that was our intent to have “replaceable” drivers. Yet it is a little more robust and better practice to have similar number of drivers and free degrees of freedom.