If you can get the air pressure lower behind the rad it encourages airflow through the rad. Hence the use of things like air dams to modify airflow under the car. Stopping air flowing UPWARDS to just behind the rad reduces the pressure there. Graham Bell describes it in his book on tuning with manometer results to show the effect. A well known tip for Morgans to improve airflow and reduce coolant temp is to block off gaps around the rad so air through the grill doesn’t bleed off round the sides. Similar with too many badges! The airdam effect is a relatively simple task using the cross brace to create the “suction effect” needed. The method described in the original post may be having a similar effect.
There is another effect caused by the windscreen....it creates a positive pressure just in front and down through the rearmost louvres, a possible aid to induction airflow/temp.
Ah...the mystique of aerodynamics...especially on a Morgan.
