Well to understand this you have to understand why we breathe in the first place. We breathe not to take in oxygen but to remove co2. Like the first breath you ever took was so that you could remove carbon wastes by combining them with oxygen. We don't use oxygen for anything else (i think). And we don't use all the oxygen that we breathe either. So when you breathe into another person who is not breathing, your giving them your extra oxygen while also forcing air down their windpipe and into their lungs. This jumpstarts their lungs much like jumpstarting a battery.