This new framework, which Apple specifically says is different from the feature in older phones, uses a combination of hardware and software to avoid performance hits as iPhone batteries age. "While performance impacts are reduced as much as possible, battery aging might still eventually lead to noticeable, possibly temporary, effects," says the support document. Apple cites "longer app launch times, lower frame rates, reduced wireless-data throughput, backlight dimming or lower speaker volume" as possible results of diminished battery health.
To be clear, the feature isn't meant to keep your iPhone's battery running longer in the short term; this is a long term strategy so that the capacity of a phone's battery stays as high as possible for as long as possible. New iPhone batteries can last for days depending on how frequently power-hungry apps are used. But after a few years, they tend to only last a day, or sometimes much less, before needing a recharge. Apple will replace an old battery with a new one, but it's either $50 or $70 depending on which iPhone model you have. Hopefully this feature will keep iPhone 11 batteries running at a high capacity throughout their lifespans.