as above warren has pretty much summed it up.

doing compressions is mimicking the hearts function which is to pump blood around the body hence transporting oxygen to organs and to the brain this alone will only transport what is in the bloodstream which is approx 6mins worth of oxygen. this is why after such time brain damage and death results (brain damage can happen after only 3mins)

when breathing our diaphram contracts and we draw air/oxygen into our lungs hence the rise fall of chest! if you stop breathing this function stops and air is not drawen/forced into the lungs anymore so we HAVE to supliment this by giving rescue breaths to the casualty which expands the lungs.

im no expert but if your trainer is teaching you as above i would be seriously worried.

its easyer to do cpr with 2 people. one gives the 2 breaths and maintains airway and the other does the compressions