Systole (when the heart squeezes) happens because the muscle cells of the heart gets smaller in size. When they get smaller we also say they "contract". Electricity going through the heart makes the cells contract. The electricity starts in the "sino-atrial node" (acronym SA Node) The SA Node is a group of cells in the right atria. These cells start an "electrical impulse". This electrical impulse sets the rate and timing at which all cardiac muscle cells contract. This motion is called 'atrial systole'. Once electrical impulse goes through the "atrio-ventricular node" (AV Node). The AV Node makes the impulse slow down. Slowing down makes the electrical impulses get to the ventricles later. That is what makes the ventricular systole occur "after" atrial systole, and lets all the blood leave the atria before ventricle contracts ("meaning squeeze"). 