Putting it on and gradually tightening the tourniquet to the point where you would sufficiently cut off blood flow past the point of ischemia and its consequent syncope, and cause a complete cerebral infarction is unlikely. More likely is that after reaching unconsciousness, your brain would become gradually hypoxic and cellular/tissue death would be very slow. You are likely to be found and rescued, wherein you will have sustained serious and irreversible brain damage.
Translation: If you can overcome the SI required to crank the tourniquet to the point where you begin to pass out (and leave it on), you will not have cut off all the circulation to your brain, but only partially. It is going to be significant amount, will make you unconscious, and eventually cause cellular death, but it may take longer than you realize to cause complete death and you risk being discovered, depending on your location.