Jam Sync   

The act of recording or poor tape quality, etc. may degrade the time code signal and make it difficult for the synchronizer to differentiate pulses. 
A time code refresher is sometimes used to amplify and alter the on/off slope of the pulses to reduce the amount of errors.  If the code disappears entirely the slave will stop unless the system provides for ‘jam sync’. 

Jam sync refers to the synchronizer’s ability to take over generating code when the source stops.  Of course the accuracy of the synchronization is in jeopardy because the new source is ‘freewheeling’ or independent of the actual code recorded - however if the drop out is for short period, jam syncing will work.