arscerebri

Whether or not it works is questionable, but the principle is simple. The air in the head is just that: air. That means that it's ~80% nitrogen, 20% oxygen (ignoring the minor components). At normal atmospheric pressure, that means the partial pressure of oxygen (ppO2) in the air-filled intracranial space is 0.20 and the ppN2 is 0.80. By Henry's law, the partial pressure of gases dissolved in a liquid (in this case blood) is a function of the gradient of partial pressure of the gas in contact with the liquid and that already dissolved in the liquid. Thus, if a liquid contains a ppN2 of 0.80 and it is in contact with air, the liquid will not absorb any more nitrogen, because it has reached equilibrium. If, however, the ppN2 in the liquid is 0.00, and it is in contact with air (ppN2=0.80), then the gradient favors the absorption of that gas into the liquid. Bottom line: less nitrogen in the inspired air --> less nitrogen dissolved in the blood --> greater concentration gradient of nitrogen between the air inside the head and the blood --> greater absorption of the nitrogen inside the head into the blood, thus clearing the volume of gas faster from the intracranial space. Any certified scuba diver should know Henry's law backwards and forwards, as it is highly relevant in diving physiology.