How do you train that? How to you structure your training, both body and mind, to have the body stable as much as possible no matter the circumstances? No matter which side of the equation it is operating on......
Gary, having answered this question above allow me to go further and add a couple of things to do with mind and thus mind stability.
Mind has a centre too and thus when centred is in a particular state, condition, which at that point is gives mind as well as body coordination.
This state of mind is one we are all familiar with and is the reason we study and practice. It is called understanding.
The second point I would make regarding the mind and the harder one to accomplish yet vital especially to Aikido is the state of mind which acts rather than reacts. The one which acts is stable yet alert and balanced and the mind which reacts is unstable and unbalanced.