It's curious that, if corporal punishment is is okay, its proponents feel the need to refer to it euphemistically. "Physical force", "physical contact" (I like that one, lol), "physical discipline". Even "spanking", really, is a euphemism, because when we use it in reference to adults it refers only to a playful activity, generally with sexual overtones.
Think: if somebody actually walked up and began striking you- an adult- in earnest, on the butt, with the intent of inflicting pain and discomfort on you, you would not call that a "spanking". It would be assault.
What we're actually talking about here is striking children.
The intensity of the blows is not really the point. It's the principle of the matter.
The location on their body where the blows are landed is also rather beside the point, in my opinion, although it does seem a bit self-defeating to strike them deliberately on the ass, while at other times instructing them that this is a "private part" and that they should tell a grown-up if anyone attempts to touch it.
I just feel there's a reason why adults are protected by law from being physically struck. By anyone, under any circumstances except in self-defense. Even a murderer on death row is ostensibly protected from physical blows to his body (on any part of his body, of any intensity).
It seems a bit schizophrenic that the majority of adults in our society believe it's okay to strike children, as long as we don't do it hard enough to permanently damage them or cause severe injury.
But that's solely my opinion.
