imo, suck out.... (exhaust system) then inside casing negative pressure, air 'll automatic come in...

juz like u walk in at supermarket toilet... they got exhaust, (prevent smell going out is 1 reason) the other is u dont need air cond in the toilet, coz as u suck out, the air cond will fill in the toilet which is in negative pressure...

so i believe the same theory can happen to casing... if u happen to got only 1 fan. better suck it out