These tendons prevent the bicuspid valve from being blown inside out. During diastole the thick muscular walls of the ventricles relax. Again, this happens to both sides of the heart. The pressure of the blood in the ventricles falls low enough for the bicuspid valve to open. At the same time, the atrium contracts.
Blood is forced from the atrium into the ventricle.
This blows the ventricle up like a balloon. The semi-lunar valves close because of the pressure in the aorta. If you have been told that your blood pressure is 120 over 90, the second figure is the pressure of the blood in your atreries when the ventricle is relaxing. The muscle of the atrium is just strong enough to force blood into the ventricle and stretch its walls. However, you will have to check with your text book for the labels to the right side.