A long time ago, Tim told me that "looseness" is not floppiness, and it's certainly not sloppiness. It's maximum freedom in all your joints at all times. Lately I'm beginning to understand part of what he meant.
Each joint of your body, be it your elbow, shoulder, hip, ankle, each joint in the spine, etc., has a natural range of motion. Maximum freedom in a joint exists when a) the muscles around the joint are as relaxed as possible, and b) the joint is at the center of its range of motion. If the joint is at one extreme or the other along its range of motion, it's not really "free" because it can't move any further in a given direction. Even if the muscles around the joint are relaxed, the lack of freedom in a nearly or fully extended or closed joint can spell our downfall, as it creates a rigid structure that can be used to disrupt balance and/or inflict damage.
How do we keep the spine free? By standing and moving as if our skeleton is suspended from the ceiling by a string that attaches to the top of our head. This creates a posture the provides maximum skeletal support to the whole body, requiring minimal muscular effort to maintain balance. It also leaves each joint in the spine at or near the center of its range of motion, allowing movement in any direction with minimal effort when necessary. After any necessary movement, return to the center like a trampoline.
The elbow loses freedom when it nears full extension or contraction. However, if someone is applying pressure towards you through your forearm, and you want to keep the muscles around the elbow relaxed and avoid offering resistance, how do you avoid closing the joint all the way?
Simple: Rather than absorbing your training partner's motion via large movement of a single joint (in this case, your elbow), distribute the motion among as many joints as possible, minimizing the amount any one joint has to move. So instead of closing your elbow all the way, the elbow moves only a little, and the shoulder moves a little, and the spine moves a little, and the hips, the knees, the ankles, etc. A little bit of movement in each joint, keeping each joint close to its natural center and hence free to move in any direction, while maintaining as much skeletal support as possible, allows a great amount of movement to be absorbed and dealt with while maintaining maximum freedom and balance and looseness throughout the motion. A big key to this is keeping the joints of the upper body free by utilizing the joints of the lower body more. This is not easy, as Tim said: "Many guys are good with their arms, some are good with their legs, but very few are good at connecting the upper and lower body together."
One might also call this body unity. . . .
By distributing all necessary motion among as many joints as possible, minimizing disruption of any one joint away from its natural center, we are better able to maintain a physical and mental state of maximum comfort, balance and nonresistance. If we can maintain that mental and physical state, regardless of what's happening to us or around us (the eye of the storm) we become strangely unavailable (movement distributed among many joints moving a little is less perceivable to the untrained eye than movement confined to a few joints that move a lot), and scarily unavoidable (we are constantly free to move in and destroy).
Remember, this isn't a RULE, just an idea to play with. The only constants are balance, looseness, sensitivity, body unity and freedom of motion.