Jason Aten, writing for Inc:

It takes a lot of energy to light up and refresh that display, so the fewer times it has to redraw what you’re looking at, the better. Other devices with high refresh rates might adjust based on what is showing on the display. For example, if you’re watching a film shot at 24 frames per second, the display might refresh at 24 or 48Hz. If you’re playing a game, it might refresh at 120Hz.

And:

On the iPhone, that’s still true, but Apple took it further by quietly included a remarkable way of deciding what refresh rate to use. Your iPhone 13 Pro or 13 Pro Max literally measures the speed of your finger on the screen, and then adjusts the refresh rate of the display.

And:

Reading a tweet, the iPhone 13 Pro drops down to 10Hz. If you start to scroll slowly, it might choose a faster refresh rate, say 60Hz. If you scroll quickly, it can ramp up to 90 or 120Hz. Apple doesn’t say exactly how many different refresh rates the display uses, only that it designed the system to match the refresh rate to the speed of your finger.

Great read.

In my opinion, ProMotion is an under appreciated feature. It works everywhere on the 13 Pro models, scrolling is smooth as glass, and there are no artifacts that hint at refresh rate changes. Beautifully implemented.