In this case cleaning involved a process of 'wetting out' and 'blotting off' in order to draw out acidic degradation products and dirt from the paper. This is done whilst the poster is face down, prior to lining. When paper is damp it is at its most vulnerable, so this treatment requires an experienced professional because of the need for good judgement about the degree of wetness, to make sure that whilst degradation products are removed, the inks are not damaged, and perhaps most importantly, to know when to stop.
Here, blotting paper is being used to clean the poster. The blotting paper to the side of the poster has been used to remove excess moisture and at the same time draw out degradation products, which are visible as a slight yellow discoloration.