I've seen four different ways to perform CPR with your hands in the past weeks.
1. Lock your arms straight, get your shoulders over the hands, and rock from the hips. So far, no surprise.
2. Perform #1 but use your abs to accelerate your shoulders downward.
3. Perform #1 but start with your arms slightly bent and straighten them as you start the downward push.
4. Perform 1, 2, and 3 simultaneously.
For those who lack the physical training and endurance to perform alternatives 2, 3, & 4, you'll have to stick with alternative #1.
A fifth alternative - not available to those performing CPR as part of their employment - is to perform pedal compressions. It's the only way I know of where the average person at home - where two-thirds of all cardiac arrests occur - can perform chest compressions from the time they witness an arrest to the time the ambulance crew is "hands on" at the victim of a cardiac arrest. See how to perform Pedal CPR at http://www.slicc.org/2014CV_AdultCPR.mov
I'm a paramedic. I've gotten a lot of CPR experience over the past decade, and I am absolutely positive that I can perform pedal compressions on someone for the ten-minute average period from the moment I see someone collapse and the time that help is "hands on." ...and I sure wouldn't bet against my being able to last for 20 minutes!
I hope this is something that you never need to know.