Every settings menu now has a slider that promises more frames for free. It is not free, and understanding what AI upscaling trades away makes it much easier to set.
The frame you see is not the frame that was drawn
With upscaling on, the game renders at a lower internal resolution and reconstructs the output. In a slow, well-lit scene the result can be indistinguishable from native. In a fast one — smoke, rain, a camera whipping round a corner — the reconstruction has less reliable history to draw on, and that is where artefacts live.
Where it helps most
AI upscaling earns its place when you are GPU-bound at a high output resolution. At 4K the saving is large and the artefacts are small, because there are more real pixels underneath. At 1080p the opposite is true: less to work from, and every mistake is a bigger fraction of the screen.
What to do with the slider
Start one notch below the most aggressive setting and look at moving foliage and any HUD element with thin text. Those two show reconstruction errors before anything else does. If they hold up, you have found your setting; if they shimmer, step back.
And if a game is CPU-bound, upscaling will not help at all — the frames are not waiting on the graphics card. That single distinction explains most of the disappointment people report after turning it on.