Afaik reflex is a bit different - it times the input to frame latency in your game, and tries to start the gameloop as late as possible in the frame so that your input gets sampled at the latest possible time and the frame still makes it to the vsync period, this isn't really a compositor tech.

Why would it need to measure input to frame latency? Surely it just needs to measure how long it takes the game to go from vsync period to frame ready and delay when the game thinks vsync happened to force it to poll later in the cycle?