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First, when you hyperventilate you expel CO2 from your system. Desolved CO2 in blood causes it to be slightly acidic so when you rid yourself of it, it leaves your blood more alkaline. Oxygen uptake into the cells is highly pH dependent and its efficiency degrades with higher pH such as when breathing fast. Second, when on your knees, or squating, your blood circulation adepts to the blood pressure increase. Once you stand up it takes a while for your system to catch on leaving you with a brief moment of low blood pressure. People with POTS (Postural orthostatic tachycardia syndrome) are very aware of these frequent moments of light-headedness. Third, compressing air into your chest cavity (breathing out but obstructing the air) is called the valsalva manoeuvre and overall decreases blood pressure further while decreasing pulse rate. Combining the rapid breathing in a sitting position followed by standing up and the valsalva manoeuvre will cause temporary oxygen deficiency in the brain causing you to pass out. For obvious reason this isn't advisable to do but if you must, then do it with someone present who can monitor you to make sure you fall safely on a soft surface and make sure you lay flat to aid blood circulation and call emergency services if you're not coming to after a few seconds and can administer CPR just in case.