SQL Server Replication and Recovery Model In SQL Server for Replication when recovery model is not very important then how it saves the transactions and also keeps the secondary instance in sync?

I tried to check on MSDN and found the following:


  Replication functions properly using any of the recovery models:
  simple, bulk-logged, or full. Merge replication tracks change by
  storing information in metadata tables. Transactional replication
  tracks changes by marking the transaction log, but this marking
  process is not affected by the recovery model


but I didn't get the concept for SIMPLE Recovery Model. And if the T-logs gets copied before checkpoint then what if someone manually executes checkpoint?
Even in simple recovery, configuring a database for Transactional Replication prevents log reuse until the Log Reader has consumed the log records and marked them as read.