A 52-year-old man with a long-standing history of hypertension presents with chest pain and is found to have a systolic ejection murmur at the right upper sternal border. On examination, the blood pressure is 160/100 mmHg, and the murmur is best heard with the patient leaning forward and in end-expiration. The murmur increases in intensity with Valsalva maneuver. Echocardiography shows left ventricular hypertrophy with a bicuspid aortic valve and severe stenosis. Which physiological principle explains why the murmur intensity increases during Valsalva maneuver?