Body-language is an important facet in our lives, whether it be business dealings or romantic relations. In our world of BDSM the importance of body language greatly increases. When disciplining or just scening with my slave, her reactions tell me so much more than her conscious blather ever could. When she is screaming "NO NO NO" but her body melts into the torment I know that she needs and craves it despite her begging and pleading.
As a training tool, being able to read the physical signs of a submissive or slave can be crucial. How they react to bondage, beatings, labor, etc provides invaluable insight into the submissive as a person and provides something of a road map for further development and training. These indicators also bring to light the slave's limits which could be, and in my opinion need to be, expressed and expanded through further training.
Taking the slave passed the point of what they think they can do or manage is one of the great pleasures of owning a slave. Doing so, though, means that the Dominant needs to be able to read the slave's reactions to know where the line is. Where pushing up to or just passed the line can be an absolutely incredible experience for both the Dominant and submissive, shoving well passed the line can be extremely detrimental or even damaging to the slave and the relationship.
As an aspect of control, pushing the slave is very important; enforcing the realization of their helplessness as property. If the slave has the ability to end a scene or stop the Dominant then the slave has some control over not just the situation, but the relationship as well. Thus, O/p and M/s relationships generally, but not in all cases, do away with the institution of safewords. Of course, the absence of safewords only exaggerates the importance of being able to accurately read the slave's body-language and reactions.
This kind of familiarity takes some time to develop, which is why I feel it is unwise to do away with safewords too soon in a relationship and why I feel there are such strong feelings in some people regarding their importance. However, experienced players within the community can usually push a submissive they just met right to the very edge of their limits, due largely to the wealth of experience and familiarity with reading these signs in general.
Knowing how to read the body-language and reactions of a slave is one of the more important aspects of owning a slave and bringing them to their full potential of servitude. More importantly, it is vital in truly knowing the slave, inside and out...