Counterpulsation. The iabp fills with helium during diastole, forcing blood in the aorta to circulate around the body and then deflates quickly during systole, making it easier for the left ventricle to eject blood.
Heart support. Another way to explain this is that when the heart is resting in between beats, the balloon inflates pushing blood back towards the heart and increasing the blood flow to the heart arteries. When the machine detects the heart about to pump, it quickly deflates, suddenly decreasing the force that the heart needs to open the aortic valve and thus allowing the heart to not have to work as hard.