*eep*
I seem to be the only one so far to vote "abuse". I would have chosen a 3rd category of both and I think others would agree based on their comments. Clearly, "it depends". However, I took the position of abuse to be devil's advocate and provide counter arguments to ponder. Shock-value seems to be the only benefit to physical aggression and although it may be necessary sometimes, it may not be mentally healthy to either party. I guess its a matter of definition, then. I think abuse is the label used when the result has a negative feedback, even if there are positive results. Discipline is guidance, rules and appropriate consequences to form or change behaviour, although the dictionary includes the word 'punishment' which connotes abuse, in my book.
A child may register the importance of the offending act more readily from a spank or may be "shocked" into snapping out of an escalating tantrum, but also learns that superiority enforced with physical trauma and pain, however slight, is condoned by the caregiver's use of it. This is a person who figures centrally on the dependant child. Some children can translate this as "monkey see, monkey do" and end up re-enacting on the playground or later in relationships. Regardless of how controlled the giver is, and IMHO control is rarely 100%, the message is that one person can dominate another using force. Indeed, a parent needs to establish this dominating role to fortify their position as the authority, however, there are other means and methods that don't need physical displays to strengthen this position. I feel the spanker can easily lose control, maybe not always, but eventually. I speak from experience, and it weighs heavily on my mind. Not to mention that it emotionally distances the two in order to be effective. It's tough to bring the child back into your trust, when the threat is present. Hence, the unhealthy affect to both.
The problem is that we're not born with the skills to deal with civilizing our children. Our innate response to disobedience is anger, and escalates to physical dominance. Even if we, as adults, have learned to control our impulses, we still need to further learn alternate ways to teach and guide our children. If we use a spank to send a message, perhaps we need to send another one in tandem to mitigate the detrimental effects. I, personally, have decided to halt the use of spanking because of my own journey to improve my communication with my son and my desire to maintain 'control' of my emotions during especially trying moments with him.