In terms of proprioception, scientists and science minded individuals tend to talk in terms of golgi tendon organs and what not.
They tend not to talk about the type of information that is actually sensed.
And this could be more important for yogi's and anyone else mindfully occupying their body since it then can make it easier to improve proprioception simply by knowing how to tune into information that is already there, or creating that information so that the brain can better feel and control the body (and so that you can too.)
If you've ever been sailing, and worked the sails, you've noticed how, when pulling on a sheet, how a strong wind working against the sails makes it harder to pull in, while working against a weaker wind is easier.
The actual act of reefing in a sail can tell you how strong a wind is just by how much effort you have to exert.
The same can be true tuning a guitar.
Turning a tuning peg while the string is slack, you can feel the peg turning with little resistance (the friction of the screw itself providing that resistance.) As the string gradually gets tighter the peg gets harder to turn and so you know that the string is getting tighter as a result.
To get further information, or confirmation of the state of the string, you could pluck it to hear how close it is to being tuned.
Work on building or truing a bicycle wheel, the older type with metal spokes, and the same idea applies. The greater the tension in the spoke becomes the harder it is to tighten the spoke further. Go around all the spokes and you can feel, just by trying to tighter or loosen a spoke how much tension is in that spoke.
If you had a torque wrench you could measure the tension more exactly, just by trying to turn the spoke using the the torque wrench.
In all the cases mentioned above, tension is measured by muscular effort.
By working to add tension or reduce tension on a sail sheet, guitar string or bicycle spoke you can feel how much tension there is. And changes in the amount of work effort tell you whether you are adding tension or reducing it.
You could continually measure the tension by continually exerting force, perhaps not quite enough to tighten or loosen a spoke, but enough that if you stop just short of actually tightening or loosening, that continual output could be used as a measurement. This would the similiar to applying a torque wrench in a similar way. You could get a torque reading or torque output.
An important factor to take into consideration, and here the bicycle wheel is a particularly useful analogy, is that muscles don't act in isolation. They work against each other (on opposite sides of a joint) or with each other (along anatomy trains, or chains or meridians of fascia that contain muscles in pockets of connective tissue between joints.
Thinking just in terms of connective tissue (lets assume that the muscles fibers stay relaxed), moving a joint in one direction creates more space on one side of a joint and reduces it on the other. This lengthens or stretches connective tissue on one side and shortens the space across which connective tissue spans on the other side.
You could think of this in terms of increasing or decreasing stretch. You could also think of this in terms of increasing or decreasing connective tissue tension (which isn't the same as muscle activation.)
With a bicycle wheel, add tension to all spokes evenly and the hub will be equidistant from the rim at all points, not matter how much tension is added. However, adjust tension in the spokes in the right way and you could displace the hub relative to the rim (or vice versa.)
As an example, tighten the spokes on the right side of the wheel and loosen them on the left and your'll slide the hub to the right relative to the rim.
Now lets say the change was driven by an outside force. Push the hub to the right while keeping the rim stable and you might just dispace the hub relative to the rim. How do you then figure out that displacement via the spokes? By the amount of tension in the spokes on the right side compared to the tension in the spokes on the left.
Lying on your back with your eyes closed, and imagining that all you can feel is changes in tension in your connective tissue (your muscles stay relaxed) if someone picks up your right leg and moves it towards your chest, at some point you'll feel an increase in hamstring tension that tells you that your leg is being flexed at the hip.
Lying on your belly and having someone bent your leg at the knee so that your heel moves towards your butt, you may feel at some point your quadriceps and hip flexors being stretched, and again that can tell you what is happening to your body.
The only problem with these two scenarios is that they require your tissues to be lengthened before you can get any sort of sense of what is happening.
And so for someone who is "tight" they'll get sensory data a lot sooner than someone who is loose.
In either case it's like driving on a road with cat's eyes. If you were using the cat eyes to sense when you go to the edge of your lane, you only sense it when you feel the tires bumping over them.
And that's the reason we need muscles as part of our proprioceptive sensor suite.
Because unless connective tissue is stretched there is little or no sensory information. And only muscle activation can create the tension in any situation.
It's like playing with your iphone. It only works if it has power. You can touch your iphone all you want but if it isn't turned on it isn't going to sense your loving touch.
If you want to turn on your powers of proprioception you need to turn on your muscles.
This doesn't mean that you ignore proprioception via stretching. It means that with muscle control you can develop your proprioceptive ability across a fuller range of movement.
Frictional muscle control teaches you how to improve muscle control and proprioception by using friction. Friction can be helpful for developing proprioception even if you are very mobile, but not very strong.
Why improve muscle control?
Muscle control not only helps you to control your body, it also helps you to feel it.
Muscle activation creates the tension that not only moves your body, but helps you to "sense" it.
With better muscle control you can use your body with less effort and make it easier to balance, improve flexibility and deal with pain and poor posture.