Click on the Synchronize toggle to place a
  synchronization marker into the recorded script file.
  Such a marker, when encountered during play of the
  script will cause the Play/Record tool to, after
  synchronizing or timeout, to reset counts of XRequests
  and XEvents, thereby establishing a baseline for
  continuation of script synchronization.

  The purpose of placing such synchronization markers
  into the script is to mark points in the script of
  known stability (no application activity).  This
  feature is intended only for advanced users of the
  Play/Record tool.