What is the reason for enhanced external counter pulsation for ischemic heart disease?

"Unloads" the heart. Eecp helps both supply and demand. It inflates during diastole, squeezing the body's arteries and causing back-pressure to push blood into the coronary arteries. It then deflates during systole, which causes negative pressure that sucks out blood from the ejecting heart, helping it eject with less effort, or strain, and hence less energy demand is required by the squeezing heart.
Improve flow. It is thought to work by reducing the workload of the heart and by improving blood flow to the heart.