A 28-year-old woman presents to the cardiology clinic with a history of progressive dyspnea on exertion and palpitations for 6 months. On examination, her blood pressure is 110/70 mmHg, heart rate 88 bpm, and respiratory rate 18/min. Auscultation reveals a high-pitched, blowing pansystolic murmur best heard at the apex with radiation to the axilla. The murmur increases in intensity with handgrip maneuver. Echocardiography confirms mitral regurgitation with a dilated left atrium and left ventricle. Which physiological mechanism best explains the increase in murmur intensity with handgrip maneuver?