By changing the shape of your body, you can change the position of your center or gravity relative to your body.
However, if you learn to feel your foundation, you can feel where your center of gravity is located, no matter what shape you make your body.
This is actually very basic physics. (Statics and Dynamics 101). And understanding this can make it easier to balance.
You can figure out the weight of a car or truck by putting the front wheels on one scale and the back wheels on another. If the weight is even on the front and back wheels (if both scales show the same weight) then the cars center of gravity is midway between the front wheels and the back.
We can use the same trick to feel (and position) our CG relative to our feet.
I'll use Center and CG interchangeably with Center of Gravity.
The blue circle shows the approximate position of my
Center of Gravity
If you feel your feet and focus on keeping the weight on both feet the same, even as you move your hips and ribcage in different directions, then your center of gravity stays in the same vertical location (directly over your feet) even as you move your upper body around. That means that you can move your body independently of your center of gravity. Or from another point of view, you can move your CG relative to your body.
In a standing side bend, with your hips pushed to the left and your upper body reaching to the right, if you keep your weight even on both feet, then your center stays in the same place relative to your feet. You know this becuase both of your feet are pressing down with equal pressure. However, because your pelvis has moved to the left, what that means is that your pelvis has shifted relative to your center of gravity.
Now what if you hold the side bend but move all of your weight on to your left foot. In this case your body moves to the left and your center of gravity moves with it.
In the same way, if you stand with vertical with your head, ribcage and pelvis all in one straight vertical line, and with your weight even on both feet, then your center of gravity is over your feet and at the same time it is somewhere at the center of your pelvis. If you then shift all of your body to the left and feel all of your weight on your left foot, then your center of gravity stays in the same place relative to your body, somewhere within the middle of your pelvis.
You could consider your head, your ribcage and your pelvis to each have their own CG.
The sum of all of these centers adds up to one center of gravity that is positioned somewhere in the region of your pelvis (if you are standing straight, with your arms by your side.)
One way to feel where this center is located vertically within your body you can lay down on your belly and lift your head, chest and legs so that you are doing locust pose. (Usually when I do this pose I focus only on lifting head and ribcage. However, for the purpose of feeling where your center is, lift legs and upper body.) You'll probably find that the your pelvis is the part of yourself in contact with the floor. For guys, unless you adjust yourself first, you'll probably find that hte pressure is least comfortable in the region of the pubic bone. So you might have to adjust yourself before lifting up.
The part of your pelvis that presses down the most is the region of your pelvis where your center of gravity is located at. If you reach your arms forwards while staying lifted, you'll tip forwards a little (or a lot.) Probably you'll find that your lower belly becomes your balance point.
If you bend your knees so that your shins point up, that shifts your weight even further forwards.
Going back to my earlier statement, not only is it your chest, pelvis and head that have their own CG, your arms and legs (both upper and lower parts) have their own center of gravity too. And the position of these centers relative to each other affects the location of the center of gravity of your body taken as a whole.
But the thing is, even though any movement of any part of your body can affect the location of your bodies CG, you can feel where your bodies center of gravity is by feeling your conneciton with the earth. Whether you are using hands, or feet or head to connect to the earth, you connection with the earth can act as a measuring device, whether scales, pressure sensors or both. And the sensory input from these sensors can tell you where your center of gravity is.
The mattress or futon is probably the worst because it moves. It seems heavier than it is. (I'm assuming its empty.) The chest of drawers is almost as bad until you either take out all the drawers or tape them shut (adding stuffing to keep stuff from sliding around.) The refrigerator is easier if you can find somewhere to grip it and you tape the door shut.
The refrigerator, and then the chest of drawers with everything taped shut and the contents muffled to prevent from moving, are the easiest to move. There is not shifting around. The center of gravity of both of these objects stays in the same place relative to the object itself.
It could be argued that the mattress doesn't have a CG. Put it on a scale and it flops down so that the edges touch the floor. How are you supposed to weigh it?
Hang it from a hook scale, then it attains a sort of shape as its covering reaches its elastic limit.
Your body's center can act in the same way. Tie all the parts together, unity by using muscle against muscle to make each joint rigid and your center of gravity stays in one place. If you become soft and relaxed and floppy, well then you are just like the mattress or futon.
Hey futon boy, stiffen up!
Because of the variable tension of our muscles, and our ability to control them, we can unify all of the separate "centers of gravity" of our body, or we can make them dissacociated.
By varying tension within our body, by varying the tension between parts, we can shift the position of the center of gravity of our whole body. This variation may only be slight. But it can be perceptable. It can add weight to one body part or be used to take it away. And of course it can be used to move the CG of one part relative to another so that the center of gravity of the whole shifts and changes.
As an example, do reclining pigeon pose with your head on the floor. Slowly lift your head. Notice the tension that builds int he back of your neck as you do so. This is your spinal erectors acivating. As they activate and pick your head up off of the floor they "add" the weight of your head to the weight of your ribcage. You may notice and in crease in the stretch to your forward leg as you do so.
Want to add weigt to your pelvis? Start with your back knee on the floor. Then use your hamstrings or glute to lift your back knee. Do it slowly and feel the addition of the weight of your leg to your pelvis. And enjoy the extra stretching that results.
How can you use this in yoga balance poses?
Say that you are doing half moon pose with one hand and one leg on the floor. You may find it easier to balance if you unify the CG of your ribcage and pelvis. How? By slightly lifting your ribcage relative to your pelvis. You can keep your hand on the floor as you do this but notice your abs engaging as you lift your ribcage. If you like, shift your weight onto your standing foot and try to slowly lift your hand off of the floor. You may find this is easier if you engage your abs first (by lifitng your ribcage a little before you take your hand off of the floor.)
An awarness that each part of your body has their own weight, their own CG can be usefull when turning your head too.
Want to look up while doing side plank pose with one leg lifted?
Then keep your head in line with your torso as you turn your head. Or focus on feeling your foot and hand so that you can feel the CG of your whole body as it shifts, so that you can then compensate for it as it shifts.
Feel your center of gravity moving back? Then shift a part of your body forwards just enough so that the center of your whole body stays in one place relative to your foundation.