On a Primary, the trepctl flush command synchronizes the database with the transaction history log, flushing the in memory queue to the THL file on disk. The operation is not supported on a Replica.
trepctl  flush  [ -limit s  ]
Internally, the operation works by inserting a heartbeat event into the queue, and then confirming when the heartbeat event has been committed to disk.
To flush the replicator:
shell> trepctl flush 
Master log is synchronized with database at log sequence number: 3622
    The flush operation is always initiated, and by default
    trepctl will wait until the operation completes.
    Using the -limit option, the
    amount of time the command-line waits before returning can be specified:
  
shell> trepctl flush -limit 1