NVIDIA Low latency - it works with the buffer for rendering frames, for example in the game OW there are dots after FPS, this is the load of the buffer, if you turn on Low Latency, there will be 1 point, if you turn off, the buffer may fill more than 1 point, which will cause a queue for rendering frames, that is waiting, which means already delayed for rendering, not for mouse input. (The video shows this process)
REFLEX is a different technology, the video explains how it works.