Valsalva maneuver increases the murmur in:

In HOCM, the gradient and the murmur is increased by maneuvers which decrease the ventricular volume such as Valsalva maneuver or standing after squatting. Factors which decrease the murmur in HOCM are by methods which increase the ventricular volume such as squatting or handgrip. Valsalva maneuver refers to forced expiration against a closed glottis. This decreases the systemic vascular return and accentuates the murmur of MVP and HOCM. This reduces the murmur of Mitral regurgitation and Aoic stenosis. Ref: Stevenson L.W., Loscalzo J. (2012). Chapter 238. Cardiomyopathy and Myocarditis. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison's Principles of Internal Medicine, 18e, Oxford American Handbook of Clinical Medicine By John A. Flynn page 52.