Fast sync works when your graphics card is pumping out more frames than your mnitor can handle.
What happens with Vsync is if your fps is over the max monitor hz the gpu halt waiting for the monitor to catchup. If vsync wasn't on, trying to jam say 90 frames a second into a monitor that can only show 60 will cause a lot of micro stutter with missing frames.
Fast sync tells the gpu to keep rendering the same frame until the monitor is ready for the next instead of gpu pausing. This reduces tearing and input lag because the gpu is still running and not stop start, stop start.