Even though multiple mechanisms operate the major mechanism is due to augmentation and diversion of blood flow towards sub endocardial region* which is main area of ischemia in most patients with Angina .
*Beta blockers does this by smoothing the myocardial contractility there by reducing trans-myocardial gradient. The coronary arterial perforators which traverse from epicardium to endocardium gets less squeezed and promotes sub endocardial perfusion.
Link to a related article from this site
Dr.S.Venkatesan MD 