The new CPR guidelines (2010) are meant to encourage untrained bystanders to help a person who collapses, and to avoid being paralyzed into not assisting by the fear of doing something wrong.
Ideally, the sequence is pretty easy. The victim collapses from a heart attack or other heart problem and is unresponsive and isn't breathing. Trying to feel a pulse fails, and the good Samaritan turns to other bystanders and tells them to go get help, and an AED (automatic external defibrillator) if available. Next comes CPR: start pushing on the chest hard and fast. In reality, a rescuer can't do much harm. Death is a reasonable definition when a patient's heart stops beating and they stop breathing. You can't get any worse.
Adding rules about technique make it too easy to have the rescuer fear, that once again, they might do something wrong.
Put one hand in the middle of the chest and begin pushing. Not too many worries with this one. Your hands have to go somewhere, and the target zone is pretty wide. Ideally, it's in the middle of the breast bone.
Compress the chest to a depth of two inches and allow it to spring back before pressing again. This is a bit problematic since it's hard to measure how far down you can press the chest. Too deep and ribs can be broken, too shallow and the heart isn't being squeezed enough to circulate blood. As it turns out, even the most practiced paramedic or nurse will break a rib or two performing CPR on a patient.