Agreed - although we always think that we breath out carbon dioxide, there's also lots of oxygen that we breath out too (40 or 60 percent of our breath, I think). This is breathed into the "victim's" lungs a few times, and then pressure is put onto their heart to squeeze it and circulate this oxygenated blood around the body as the heart would normally do if it was beating.
It's apparently quite rare (outside of Hollywood) for someone to regain consciousness as a result of doing this alone. If you're ever unfortunate enough to be performing this, don't expect the person to sit bolt upright after a while and spit out some seaweed/seawater and cough lots. Especially if they're not near the sea.