The "wash out" isn't from the mask, it's from your body... even 4cm pressure is typically enough to clear any exhaled breath from the hose and mask, and higher pressures will cause more venting, and even faster clearance, but, it's all gone before you take another breath in.
As Pugsy says, the higher pressure differential between exhale and inhale pressure causes (slightly) deeper breaths, or, more ventilation of your lungs, and that extra ventilation 'washes out' more carbon dioxide. EPR gives up to a 3cm difference between inhale and exhale pressures.
Carbon dioxide in the blood affects your blood's ph, causing it to be more acidic. As you breathe out more co2, your ph goes up just a bit, (becomes less acidic)... The more acidic in your blood, the stronger the urge to breathe is.
This loop cycle is what drives your unconscious breathing, and some people are more sensitive to it than others... with a minority of people, (maybe 1 in 7 (depending on which study you look at)), just a LITTLE drop in the co2 and they don't feel the need to breathe for a some seconds, sometimes long enough to be counted as an apnea, and since their airway is open, it's a central, not obstructive apnea.
So, when we see central apneas in groups, not just one here or there, the first thing to do is reduce the ventilation, down to what the lungs do naturally, and see if that lowers the central apneas.
