A brain cannot live if it cannot control its body, and it cannot control its body unless it can perceive the body's spatial configuration. The good news is that perception is a far simpler problem than control. Yet it involves the deepest laws. How might proprioception work?
Proprioception is vibrational tomography. In the simplest form, proprioception is like sonar: first active signals from the brain ("pings," or muscle-fiber firings) create vibrations, next passive sensors (hydrophones, or mechanoreceptors) collect the pings' vibratory echoes, and finally a tomography algorithm reassembles them all into a 3-D representation. This is not a hypothesized model of how proprioception might work, but a description of how it must work, because a body has no other information sources to use (vertebrate joints, unlike robot joints, contain no angle sensors). The only question is how the tomography is implemented.
A body is an active resonator containing low-entropy attractors. At the finest level of detail, proprioceptive tomography must involve coordinating muscle-firings (and their echo filters) to maximize bandwidth and signal-to-noise ratio. In other words, active proprioception creates low-entropy attractor states (or "eigenmodes" in the colloquial sense) among muscle firings and vibrations.
Furthermore, as a gross mechanical system, a body must have passive resonances anyway (albeit highly damped resonances, with low Q). The coherence-enhancement discussed above amplifies body vibrations, raises their Q, and makes them self-synchronize, so that an actively-controlled body will naturally tend to "ring" with its dominant eigenmodes. Because body-control is a brain's single most important task--in terms of evolution, bandwidth, response latency, and computational difficulty--these eigenmodes will be the highest bandwidth and most critical signals reverberating inside a body and between fellows.
At the low frequency end are the postural eigenmodes, like the basic arching, coiling, and swimming motions of a snake, or the various whole- body gestures instinctive to bipeds: chest-puffing, skulking, cowering, and so on. At intermediate frequencies would be eigenmodes corresponding to trans-sensory biophysical state (an informational description for what would colloquially be called emotions): confidence, trepidation, attraction, etc.