Application of a magnet can successfully terminate pacemaker-mediated endless loop tachycardia in DDD pacemakers by rendering the device insensitive to incoming signals.
Implantation of an AV universal DDD pacemaker, stimulating and sensing both atrium and ventricle caused a pacemaker mediated tachycardia, begun by a P wave occurring after a premature ventricular contraction, and sustained by P waves caused by retrograde AV conduction. The tachycardia was terminated by application of a magnet to the pulse generator which temporarily made it insensitive to incoming signals, both P and QRS.
Furman et al. (1982) studied this question.