Here's two BSFC plots with horsepower isolines:
Any of the points on an isoline produce the same horsepower and get the job done just as well. Suppose you need 27HP (20KW). The best way to get it is with lots of throttle and low revs.
These plots, especially the ones with HP isolines overlaid, explain why P&G saves gas. Suppose you have a manual tranny and the 1.5L Prius engine from the second image, and it needs 10kW to cruise down the highway. You're probably going to do that at 2000 RPM and half throttle. If you have enough gears, you can save some more gas by cruising at 1500 RPM and 3/4 throttle. But if you can efficiently make use of 30kW (40HP) at full throttle, then switch the engine off and coast for a while, you're producing the same average power with less gas.
Here's an extreme engine whose sweet spot happens to produce quite a bit of power. P&G ought to use fully 10% less fuel:
