This is known as the rapid ejection phase of ventricular emptying. The blood pressure in the ventricle and aorta rise due to the continued contraction of the ventricular myocardium and the rapid increase in the volume of blood in the aorta. This distends the aorta. Ventricular pressure drops below aortic pressure due to the relaxation of the ventricular myocardium. Blood pressure in the ventricle drops due to relaxation of the ventricular myocardium, but ventricular pressure is still higher than atrial pressure.
Ventricular pressure drops below atrial pressure and the mitral valve opens.
Blood begins to flow from the atrium into the ventricle. Ventricular pressure continues to drop because the myocardium is relaxing faster than the ventricle is filled by blood from the atrium. The first 0.06 seconds after the valve opens constitute the Rapid Ventricular filling phase. At the end of this phase, ventricular pressure begins to rise as the ventricle starts becoming distended. A large fraction of ventricular filling occurs during the rapid filling phase.