Why is the murmur of hocm louder on valsalva maneuver?

Anything : That decreases venous return leads to a reduction in volume to the heart. These changes will effect the intensity of murmurs. With the valsalva there ia a transient/ brief increase in output. The straining portion decreases venous return and will increase the intensity of the murmur.Heart rate will also increase. Standing abruptly will do the same.