Here's the most important thing I understand to this date (July 31th, 2019) in regards to helping people improve quality of movement, reduce pain, and continue to train and make gains pain free:
Muscles move bones.
If you cannot feel a muscle, you need to visualize organizing your bones to create the muscle activation you desire. If you don't feel your hamstrings during a deadlift, you need to pair your hamstrings with your abs to stack your pelvis under your rib cage and flex/extend with a stable trunk. If you can't feel your glutes when you squat, you need to envision your hips rotating back into your glutes so they can load up like a rubber band.
The second most important concept that I understand right now is:
The Human body is meant to rotate.
We can assume someones quality of movement based off their quality of rotation at the various joints of the body. For example, the spine, shoulders, hips, ankles, all have a normal range of motion in the transverse plane. When these motions are assessed, whether or not someone is competent in the sagittal and frontal plane of motion will be apparent because transverse plane motion will be available, or it won't.
If someone is limited on both sides of their body in the transverse plane, they are not competent in the sagittal plane.
If someone is limited on one side of their body they are not competent in the frontal plane.
In both of these situations the person in front of you is not competent in the transverse plane. As discussed in #1 and #2 of Tri-Planar Consideration series the body organizes via a hierarchy. First we flex or extend a joint to enable or limit frontal plane motion. Once frontal plane musculature is engaged, we are able to stabilize and then rotate onto one side of the body at a time.
Why is this so important? We do almost everything with one side of our body at a time. Walk, run, climb, jump, and crawl. All require our bodies to stabilize on one side of the body at a time. These are all variations of ambulation but this concept transfers to weight lifting, sports, and normal everyday activities.
Without transverse plane access (and competence) we as organisms are lesser beings. We lose resiliency, comfort, and there is even studies to show that emotional states can be affected. Rest and recovery are not as easily acquired because a body that does not rotate does not exist in the "in-between" phase of motion. Therefore, one phase of motion is always on. This could be flexor muscles, extensor muscles, left rotators, right rotators etc. If those muscles are not releasing into a long position and allowing for complete motion of the opposite movement, there will be a degree of overuse and under use in the respective muscle groups.
Accessing the transverse plane and building competence requires competence in the sagittal and frontal planes. If you aren't familiar with how to build control in the first two cardinal planes of motion, refer first to here for sagittal, and then here for frontal. Once you are able to identify competence by muscle recruitment during frontal plane exercises, it's time to step it up to the transverse plane.
There's no rotation like counter-rotation. Humans are like wind up toys. We throw one arm and the opposite leg out in front of us and then reverse it. Boing-boing down the street we go. Ambulation was the most important characteristic to our survival for thousands of years. Therefore, everything we do as humans as enabled us to be expert ambulators. However, in today's day and age, where food, water, and shelter are easy to come by; we have eliminated a majority of our need to ambulate. We sit in school. We sit to travel. We sit for entertainment. We have replaced our dominant ambulatory nature for the seated posture. Overtime, nurture begins to conquer nature and we lose our ability to rotate because sitting is a sagittal plane task.
That is why we need to retrain the transverse plane.
If this exercise has you falling over backwards, you are not ready for transverse plane. You must first have adequate frontal plane control before accessing and training the transverse plane.
Retro walking, counter rotation, hip internal rotation, hip external rotation, trunk left rotation and right rotation is the epitome of human nature. Rotation is human. We need to be able to access our hips if we are going to generate power. We need to rotate our trunk if we are going to express power. Look at the baseball swing, the golf swing, the sprinter, or football kicker. Humans need to rotate in order to ambulate, hunt, move objects, build shelters and pump their muscles through normal and full ranges of motions.
By moving the body through a full range of motion we can compress and decompress joints. We can expand on side of our lunge, and spine, and hips, and shoulder, and compress the opposite side. We can move lymphatics and "drain the body of toxins."
The transverse plane is what enables us as humans to be the most superior beings on planet earth. It helps us stay balanced and upright to see farther than chimpanzees, run farther than lions, and squat lower than gazelles.
If you are missing motion and control in the transverse plane I have one piece of advice for you. Find out why.
Once you understand why you are limited, why you are in pain, or why your performance isn't what it can be. You can do something about it.