Downsampling works by getting your monitor to resample a higher resolution image to fit the screen and a by-product of this is an anti-aliasing effect. For example, if your native resolution is 1080p you could tell your game to run at 1440p and then your monitor will do the 1440->1080 conversion. To do this you need to use a tool to add the new resolution to Windows. For Nvidia cards the tool is included in the driver control panel but AMD/ATi owners need to download an external tool. The effect is hard to show in screenshots but it definitely looks great in motion and the best thing is that it works on every game. Obviously the drawback is the performance hit of running at the higher resolution but if you’ve got a game that runs well you can get really nice image quality from downsampling.
^^ IL2 downsampled from 2560×1440 to 1920×1080