Denoising Animations on November 2018 Brecht und Lukas Stockner introduced a way to Denoise an animation after it is rendered. They are using more than one frame to produce a flicker free image.  

https://developer.blender.org/D3889

Does anyone know how this works or could provide an Manual?
I think you are talking about the Denoise node in the Compositor.

https://cgcookie.com/tutorial/denoising-renders-in-blender-2-81

https://developer.blender.org/D4304