What are the two primary theories of why hands-only cpr is effective?

Hands-only CPR is said to be effective because of the two primary theories. First, a recent studies have shown that the hands-only CPR is just as effective as the old ABC version of CPR.  They found out that the rate of survival and brain damage between the two groups who perform traditional CPR and hands-only CPR was remarkably similar.  Second, any bystander can do the hands-only CPR for it is such a simple method that helps maintain brain and heart perfusion while waiting for a much better help. In essence, immediate help is crucial especially in treating cardiac arrest and hands-only CPR is an effective way in saving life.