From my limited testing and basic understanding, it seems to cache one of the higher cascade resolutions and renders it at a further distance using the cached data. I have attached 2 images from my test. Enabling the setting actually reduced my FPS very slightly, but the shadow quality went up. My drawcalls and triangle count also reduced, which is actually the same effect you would get if you reduced the number of dynamic shadow cascades on your directional light.
Note I am on 5.5.4 using forward rendering and this is only 1 datapoint, so your results may differ.
