First, this is the inability to perceive chronic holding and misalignment in one's own body. Perception requires movement, and so holding and decreased motility tends to fall out of consciousness. Second, it is the tendency for the senses not to directly perceive in others, what one cannot perceive in oneself.
For instance, if ones back is holding a lot of tension chronically, it will be hard to notice the tension that others are carrying. Likewise, if one is not vibrating much, there is an inability to perceive the vibration of others. If one's own voice is monotone, it will be hard to perceive subtle inflections in someone else's voice.
The combination of decreased body awareness and perceptual ceiling can make bodywork a bit of an act of faith in the beginning. It can be difficult to understand what is being asked, and also doubts can arise about whether anything is actually happening, or whether goals of bodywork are real.
This contrasts to complex volitional behaviors, which are discernable in others even to those of us who do not act that way. For instance, a shy person sees outgoing behaviors in others and does not doubt they occur. It is much more graspable to try to imitate such behaviors. This falls short in the end because the behavior is not natural.