code: public void scheduleBlockReport(long delay) {
      if (delay > 0) { // send BR after random delay
        lastBlockReport = System.currentTimeMillis()
                              - ( anode.blockReportInterval - R.nextInt((int)(delay)));
      } else { // send at next heartbeat
        lastBlockReport = lastHeartbeat - anode.blockReportInterval;
      }
      resetBlockReportTime = true; // reset future BRs for randomness
    }
docs: This methods  arranges for the data node to send the block report at the next heartbeat.