Did mine whilst in the services these are done annually and it's driven home to you until it's second nature, also did the Canadian forces training course much more intensive and included skiing etc.
... The reason, they said is that the (british medical journal I think?) believe that chest compressions in no way at all are compressing the heart, its to deep in the chest, and what the compressions are actually doing are pushing blood and air in and out of the lungs, thus pumping the heart by proxy. ...)
The second time I gave CPR was interesting and unusual.
While I was giving CPR to a yachtsman, the ambulance crew arrived and attached the defibrilator with ECG.
When I backed off they gave the first shock. On the second shock, there was a big flash and the debfib was kaput!
However the ECG was still working and displaying the fibrillation
As I resumed compressions, the ECG display recorded each compression with a spike in the otherwise fibrillating wave. Similar to that recorded by a normal heart.
The paramedic felt a firm pulse in the carotid artery.
So did the surgeon who was monitoring the groin pulse.
Until that day, I always felt like that compressions were a waste of time if no result was achieved in a few minutes.
If you do it right, you become that persons heart.
Never give up till you are relieved especially on children.
I'm currently saving, very slowly, to do an ExMed type expediction Medics course. You can't know enough in my mind.