Paroxysmal supraventricular tachycardias (PSVT), including atrioventricular nodal re-entry tachycardia (AVNRT), are commonly treated with vagal maneuvers. The modified Valsalva maneuver, combining forced expiration with postural change, has shown higher success rates. A 44-year-old woman with recurrent PSVT underwent a modified Valsalva maneuver under ECG monitoring. The maneuver induced a 10-second asymptomatic polymorphic VT that resolved spontaneously. Echocardiography and labs were normal, and electrophysiology confirmed AVNRT. The patient was successfully treated with cryoablation. The modified Valsalva maneuver is generally safe. However, in rare cases it can induce ventricular tachycardia through autonomic, hemodynamic, or re-entry-related mechanisms. The modified Valsalva maneuver remains a first-line treatment for PSVT, but initial use should be ECG monitored. If polymorphic VT or other ventricular arrhythmias occur during the maneuver, further electrophysiological evaluation is warranted, and ablation should be considered for confirmed PSVT.