A supple spine is one that can move when we want and solidify when we want. The movement part is relatively easy for most in at least some areas of the spine—but the solid part is another story entirely.
We can look at the body as three sections that work together—the arms, legs and the trunk. For different reasons at different times we want these sections to be fixed or mobile. Finding solidity in the arms and legs is a lot easier than finding this same solidity in the spine.
The upper extremity is made up of three bones with one joint—the radius and ulna make up the forearm and the humerus is the upper arm.  To make this a solid pole requires effective use of the elbow joint to straighten the arm. The lower extremity is also made up of three bones  with one joint(the arms and legs are mirror image of each other in certain structural aspects)—the tibia and fibula are the bone of the shin and the femur is the massive bone of the thigh. Again, all it takes to make the leg effectively solid is making proper use of the knee joint.
Hyperextension of the knees or elbows is the example of improper use of these joints. You can bear weight through hyperextended joints it just won’t transfer as successfully as if the joint is properly extended.
As opposed to these one joint poles, the spine has twenty five bones with twenty four joints connecting it together. When we are born the spine is anywhere from thirty two to thirty four joints but the tailbone which is anywhere from three to five bones, and the sacrum which is five bones, both fuse at some point in infancy into two separate bones.
The ability to bring the same solidity to the spine as in the extremities requires a great deal of core tone in the pelvic floor and the abdominal muscles. While it is probably pretty clear that we would want this solidity of the spine in plank position or any core work— we also want this same solidity with every step we take.
I exercise to compliment my movement patterns. All work I do on my body is to bring it closer to its functional design and walking is one of its core functions. Walking with a solid and connected spine allows for the most effective use of whole body. Arms and legs that radiate from a stable spine are much more likely to work as designed.