Nvidia reflex works by inserting a wait before input processing that dynamically adjusts so that the frame completes rendering right before the monitor's v-blank. Thereby reducing the amount of time the completed frame sits idle in the backbuffer, which in turn reduces the delta between when input is sampled and when the frame is displayed.

To do this effectively is very difficult without a complete view of the pipeline all the way to the display. That is why a feature like that works best when the driver and game engine work together rather than either side attempting to implement it unilaterally.

It seems that AMD attempted to implement this feature without help from the game developer by hooking the Windows input APIs directly.