I am afraid that the you tube video is rather misleading. The man who made it has confused manifold vacuum with carburettor airflow . The two are not the same at all. Manifold vacuum can be affected by a whole array of factors e.g. tappet adjustment on the cylinders it feeds, and what he is doing is equalising the vacuum between carburettors. In any event, with most multiple carburetter setups, including the Rover, there is a balance port in the manifold which automatically will equalise the vacuum , and to a lesser extent, it also affects the airflow i.e. any given cylinder draws mixture from both carburetters, albeit to differing degrees. To synchronise carburetters properly, the depression must be measured on the atmospheric side of the butterflies