p 189 Rapid Review Path
"Aortic regurg is a problem in closing the aortic valve. Because the valve closes in diastole, the murmur occurs in diastole. It is characterized by a high-pitched "blowing" early diastolic murmur immediately following the S2 heart sound. It increases in intensity on expiration and is heard best along the left parasternal border"
My understanding from PD class is that this point (erbs point) upper part of the left ventricle is(right by the opening to the aortic valve, so the aortic valve is actually just behind the left side of the sternum at the 3rd intercostal space (see page 205 of Drake's Greys anatomy for students). We listen to the aortic valve at the right second intercostal space because that is where the blood is normally ejected into the aorta, so it is heard best there. In the case of aortic regurg the blood is flowing back into the left ventricle and therefor the murmur can be heard best at the 3rd intercostal space on the left of the sternum(erbs point), because this is the direction in which the blood is flowing in the murmur.
Its just like listening to the valves in a normal heart, where it can be heard best is where the blood is flowing to. The aortic valve is heard best at the right 2nd intercostal space because the blood is being ejected into the aorta, the pulmonic valve can be heard best in the left 2nd intercostal space because the blood is being ejected into the pulmonic artery. In aortic regurg the blood is flowing back into the L venticle and can therefore be heard best at the 3rd intercostal space on the left sternal border.
Oops guess i repeated myself, but I hope this makes some sense and helps