I think the phrase used to be "spare the ROD".... So what the hell was the "rod" they were referring to? A curtain rod? lightning rod? rat rod?
Well... It was the BEATING rod, of course! Back in the day, parents used the beating rod to teach the children to mind them and it worked. Usually, it only took one good beating. Children tended to remember that and the mere sight or mention of the beating rod was enough to strike fear in the hearts of the misbehaving child. Now this all sounds really brutal but they never had any problem with ADHD. The beating rod cured that.
Now, I am 55, so I came along after the beating rod became out of vogue. My father used the good old leather belt, which wasn't quite as barbaric but had the same effect. What bugged me the most was, my sisters never got the belt. Seems like, even when THEY did something wrong, the blame was somehow cast on me, the older sibling, and I got the belt. As we became older, the belt was no longer used, we were simply "grounded" for an indeterminate period of time. Grounding was somewhat ambiguous, it could include any number of restrictions on our liberty. As a teen, it seemed to be almost as undesirable as the belt. When you were grounded there was no TV, no radio, no telephone, no games to play. You didn't get to go out and play, and it usually involved some obscure chore that no one ever wanted to do, like clean the basement. When the grounding sentence was handed down, it was never for a specified time, it may last a week or a month, you never knew. It largely depended on your behavior.
In any event, none of this happens nowadays. Children rule the roost in most families. Beating rods, belts and even groundings are viewed as "child abuse" and subject to litigation and/or criminal charges. We now have a few generations of children who have been raised in such environments, you be the judge as to whether or not that is for the better.