The shape of the clutch bell housing in 6-speed 1.8Ts, counting as well the fact it has two holes (
lower hole for water egress and top hole for timing check) and the very small, 2-3mm free play between flywheel teeth and housing, is very similar to a
rotary vane pump.
Checked in practice: removed rubber plug from the timing hole, let the engine turn while cold (fast idle), the pumping of the air outside through the timing hole can be felt by hand from 1200rpm upwards. Obviously, at 3000-5000rpm the pumping rate is far greater.
So the Leons which have the full plastic undertray (1.9TDI standard, other Leons by retrofit) to avoid the ingress of water, mud and dirt from road can run without rubber plug in timing hole, and therefore the flywheel sucks fresh air from below and pumps it upwards, cooling itself better.
With rubber plug in place, it churns the same air again and again and cooling is much slower.