http://docs.mongodb.org/manual/core/backups/#replica-set-backup-considerations

The lines:
"It is possible to lock a single secondary database and then create a backup from that instance. When you unlock the database, the secondary will catch up with the primary." is incorrect as replication does not pause during a secondary being locked.

When a server is fsync and locked, reads are blocked the moment a write comes in. This is documented in SERVER-1423 with replication mentioned in SERVER-4143.

The line should, therefore, be removed. The current recommended method for doing a point in time backup is to use the --oplog option with mongodump which will provide a point in time backup similar to doing an fsync lock and unlock.