Marcel,
This ia a good graphic and it just about sums it up!
Patrick, as you know, warm air flows up as heat rises, in through weap holes joining air from soffits which grabs all humidity it can carry as it races to the exterior through the maximum which by itself creates a chimney effect.
All this is gravitational and non mechanical.
If lateral vents are not blocked, then the air goes through laterals and out through the maximum leaving walls and soffits non ventilated, therefore the conditions may deterriorate, the roof surface will not last as long and so on!
Salut, 