I can’t tell if this is a troll post or not, but either way, the concept the author seems to be looking for is called the “Law of Demand” [0].

0: https://en.wikipedia.org/wiki/Law_of_demand

No, the concept the author is talking about is the Jevons paradox, just like the title says.

The law of demand frames demand as a function of price and utility — demand is monotonically non-decreasing with utility (the more useful it is, the more people want it), and monotonically non-increasing with price (the pricier it is, the less people want it), but e.g. Giffen goods and Veblen goods break the "monotonically non-increasing with price" assumption of the law of demand.

You can add efficiency to that equation — demand is a function of price, utility and efficiency, and it is also monotonically non-increasing efficiency (The less of it you need, the less people want it). If you could get twice as much saltiness from table salt, you'd cut down demand by 50%.

The Jevons paradox is about the cases where demand isn't non-increasing with efficiency, because utility is itself a function of efficiency. Increased efficiency directly lowers demand, but, because it increases utility, it also increases demand indirectly.

The paradox is usually framed as more efficiency -> more demand (because of the intermediate "more utility" step), but the author is framing it in the opposite direction, as less efficiency -> less demand (because of the intermediate "less utility"). I would argue it's just that the paradox works both ways, rather than calling it a "reverse", but that's me.

Not a troll. As recently as the 1870s, when Jevons, Menger, and Walras pioneered the Marginal Revolution in economics, this was a deep new insight about elasticity.

Imagine yourself a shop keeper, hoping to boost the money coming in at the till. If you increase prices by 10%, you will get more? Right?

That depends on the elasticity of demand. If the elasticity is two, the drop in demand is twice the increase in price. 0.8 times 1.1 is 0.88. Takings fall from $100 to $88.

But if the elasticity is one half, the drop in demand is half the increase in price. 0.95 times 1.1 is 1.045. Takings rise from $100 to $104.5.

When the price goes up the shop always sells less goods, (Law of Demand) but that still leaves it unclear whether more or less money goes in the till. This is first year University economics today.

Back in 1865, it was obvious to every-one that the increased efficiency of steam engines would lead to a reduced demand for coal. Jevons pointed out that increased efficiency makes steam power cheaper. Goodbye water wheel, hello steam engine. More steam engines, greater consumption of steam power, any-one who wants to make a prediction needs to invent the concept of elasticity and try to measure it. Greater than one? Less than one? That is going to decide whether total demand rises or falls.

exactly the Law of Demand, it is more intuitive