So going back to the original post, I think there are a few assumptions that need to be addressed.
First, yes, obviously lowering a yield is “manipulation” as is everything else done to make wine. But the idea of lower yields increasing the grape quality is not a given. As some have pointed out, the yields can be lowered in many ways but I think even that misses the point.
So where did the idea come from? Back in the day, you wanted to make the most wine you could. You were a hard-working farmer and you certainly didn’t want to have insufficient wine or potatoes or wheat or whatever it was you were growing. So you pushed for as much juice as you could get.
But some people noticed that there was some correlation between the tonnage of a vineyard and the wine quality. If you grow vines densely on low-nutrient soils, you restrain the natural vigor of the vines and consequently people placed limits on yields. They had no way of really measuring what was going on however. And it’s a very crude mechanism to improve quality and it’s counter-intuitive. Why? Because if you are in the 4H club and you want to grow a prize pumpkin or melon, you eliminate the other fruits so the vine puts all of its energy into that one fruit. Same thing if you’re exhibiting roses.
Then why green-harvest?
To counter all of the other manipulation you’ve subjected the vines to. That goes to the way in which a growing plant operates.
Woody plants store carbohydrates in their woody parts and roots. Those reserves are what the plant uses when it breaks dormancy in the spring. Even more importantly, most of the potential for fruit for next year is set this year. So conditions this year place a limit on what kind of crop you get next year. But what do people do? They prune away much of the plant, except for one or a few canes.
If you cut off the canes and train the remaining ones into an unnatural shape, you manipulate the plant into doing things it wouldn’t normally do. It may have the buds to produce more fruit than it has canopy to ripen. And if you prune too early, you may end up with a second set of buds for fruit, which means you decrease the quality of the wine.
One place green harvesting works is when you have low-nutrient soils and your pruning can trick the vine into directing energy into ripening its now-reduced crop load. But if you have hi-nutrient soil you may end up triggering additional vegetative growth, which isn’t what you want. So whether you reduce yields by green-harvesting is at least partly dependent on the vine and the vineyard.
Most importantly, you need a certain amount of canopy to produce a certain amount of acid, sugar, etc., and to ripen a certain amount of fruit. The ripening process has it’s own timetable. Simply “green harvesting” willy-nilly isn’t the answer because it may cause exactly the opposite of what you want - if you green harvest at the wrong time, you end up with bigger, juicier, and more diluted grapes, or excess vegetation.
So you reduce the yields thru green-harvesting when you’re accommodating the vine to the specific conditions you’ve placed it in, which may in fact be designed to further reduce yields - whether that be dense planting, low-nutrient soil, or some trellising or training system. And don’t forget, you can select rootstock to de-invigorate the plant as well.
Finally, the idea of lowering yields to produce better wine is a misfire. For There’s no reason you can’t actually increase quantity with no loss in quality. For each vine in each section of the vineyard, there’s an optimum ratio of leaf area to “perfect” ripening capacity. As noted, that may actually change from year to year. Rather than the crude tool of “lowering yields”, whether by vineyard or by vine, I think it’s more logical to try to figure out how you can get the largest possible amount of perfectly-ripened grapes, which would include clonal selection, rootstock selection, trellising systems, irrigation, row orientation, density, and so on. Every one of those choices is a manipulation. That’s why I have to disagree with Keith, who otherwise makes a lot of sense - it’s fact, not sophistry.