PS5 Pro adds upgrades in shadows, screen space reflections (SSR), screen space global illumination (SSGI), and screen space ambient occlusion (SSAO). Here's how a single frame breaks down into its constituent parts, in particular showing the direct impact of the SSGI light bounce between characters and the environment.
The result is more frames per second than would otherwise be possible. In most scenarios on my own VRR display, frames are actually output at anywhere between 60-120fps, but typically performance hangs between 80-90fps during the opening mission. Only one major drop, briefly under 60fps, is recorded while exploding a planted C4 on the open world missions. Beyond this, the VRR mode genuinely works, and the team claim it to be more responsive than running the game unlocked with the game's actual 120Hz output mode active.