Fibrillation in the heart often consists of multiple spiral waves of electrical activation in cardiac tissue. To terminate these multiple waves, recently proposed Low Energy Antifibrillation Pacing (LEAP) uses a series of low energy pulses. This achieves an energy reduction of about 80textpercent in animal experiments. To understand the mechanism of LEAP we study the interaction of electric pulses with pinned spiral waves in monolayers of cardiac cells. Optical mapping and controlled placing of heterogeneities allow us to observe the activation dynamics in these monolayers during field pulsing. We show that a pinned wave can be terminated by a series of pulses when one of the pulses falls in the vulnerable window of the pinned spiral.