The ejection systolic murmur in hyperophic obstructive cardiomyopathy is diminished when a patient -

Ref:Harrison&;s 19th E pg 1568. This increases venous return. And hence amount of blood in LV. In HOCM the blood acts as a physical barrier between hyperophied septum and anterior mitral leaflet. As the patient lies down, venous return increases, fuher increasing the blood coming to the ventricles. This blood prevents the hyperophied septum coming in contact with the anterior leaflet of the mitral Valve. Hence murmur reduces.