We purchased "The Rod" several years ago. It's a this fiberglass rod witha large, imposing foam rubber handle.
The first thing I did was to whale on the front of my leg to see how hard it was to hurt. It takes a lot to do more than sting.
Discipline using corporal punishment is not designed to inflict serious pain. It is to use the judicious use of pain to train a child to modify his behaviour.
We rarely used "The Rod." The mere threat was enough to change behaviour well over 90% of the time.
Right on target. Fear of spanking has to be backed up by the possibility of the real thing, or it's meaningless.