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

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?