Jam sync refers to the practice of applying a phase hit to a system to bring it in synchronization

Synchronization

Synchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....

 with another. The term originates from the use of this technique to replace defective time code

Time code

A timecode is a sequence of numeric codes generated at regular intervals by a timing system.- Video and film timecode :...

on a video tape recording by replacing it with a new time code sequence, which may be an extension of a previous good time code sequence on an earlier part of the source material .