To sum-up, it's not directly related to G-SYNC operation (the operation of which primarily occurs between GPU and Display), but instead to the pre-rendered frames queue (which occurs between CPU and GPU); e.g. Low Latency Mode (formerly known as "Maximum Pre-rendered frames" in the NVCP).
Battle(non)sense has released an explainer video with tests:
1. For games that support Reflex, LLM and Reflex can be stacked, but LLM will have no further beneficial effect in this case, and can be disabled.
2. Reflex "On" = (true) MPRF "0" (even in GPU bound scenarios), Reflex "On + Boost" = (true) MPRF "0" (even in GPU bound scenarios) + Power Management Mode "Prefer Maximum Performance."
3. Reflex eliminates the pre-rendered frames queue, but it does not prevent sync-induced input lag or keep G-SYNC in range, thus a minimum -3 FPS limit (be it external or internal) is still required to remain in the G-SYNC range. Same goes for low lag V-SYNC; an FPS limiter is still required. EDIT: With G-SYNC + NVCP V-SYNC + Reflex, it has been suggested in Nvidia documentation that Reflex behaves like LLM "Ultra" in that it sets an auto FPS limit slightly below the refresh rate. Once I've had a chance to more fully test Reflex myself, I'll ultimately amend my optimal G-SYNC setting (https://blurbusters.com/gsync/gsync101- ... ttings/14/) with general information and recommendations (below the existing LLM section) for Reflex configuration when used with G-SYNC.
Update #1 (10/03/2020):
I've now tested and verified base Reflex functionality in Call of Duty: Warzone, Fortnite, and Valorant...
- Nvidia Driver 456.71
- XB271HU (144Hz native G-SYNC)
- 1080 Ti
- Reflex applies an automatic FPS limit slightly below the refresh rate. At 144Hz, it limits to ~138 FPS, just as LLM "Ultra" (in supported games) did before it. This auto FPS limit is relative to the current maximum refresh rate.
- If an in-game or external FPS limiter is to be used with Reflex enabled, the in-game or external limiter must set slightly below Reflex's automatic limit for it to override Reflex's. This will not override Reflex's auto FPS limiting function (and thus, render queue elimination benefits) when the game is GPU-bound and frames are fluctuating, and will instead only override the initial static Reflex FPS limit where the game is not GPU-bound and the system can maintain a framerate that could otherwise exceed the limiters if they were disabled.
- Reflex does not apply an automatic FPS limit slightly below the refresh rate, but retains all other functionality.