What prevents the backflow of blood from the aorta to the left ventricle?
When the left ventricle contracts the ventricular pressure increases rapidly and aortic valve opens. The contraction is called systole.
Next, when the systole ends and the left ventricle stops contraction (this is called diastole) the aortic valve closes with a snap. This closure prevents back-flow of blood from aorta to left ventricle and produces the second heart sound.
This diagram shows the position of the aortic valve :