PMT technically refers to any tachycardia induced by a pacemaker, but most often refers to a re-entrant loop tachycardia where the pacemaker completes the anterograde loop of the re-entrant circuit.
PMT is often initiated by a non synchronous ventricular beat (e.g. a PVC) that is conducted retrograde to the R atrium. Because the atrium and the ventricle are “out of sync”, this retrograde atrial depolarization may fall out side of the atrial blanking and the Post Ventrial Atrial Refractory Period (PVARP). The pacemaker cannot distinguish a retrograde depolarization from a spontaneous polarization and it will trigger a ventricular paced beat after the programmed A-V delay. That would complete one loop of the re-entrant loop.
If the paced ventricular beat is again conducted retrograde, and it again falls out side of the PVARP, the cycle will continue – potentially for a long time (however most modern pacemakers have algorithms that can abort PMT, and there are easy things you can to in the ER or in the pace clinic to abort PMT.
PMT tends to be relatively fast and is bounded by the programmed upper tracking rate.