A checkpoint occurs when the DBWR (database writer) process writes all modified buffers in the SGA buffer cache to the database data files. Data file headers are also updated with the latest checkpoint SCN, even if the file had no changed blocks.
Checkpoints occur AFTER (not during) every redo log switch and also at intervals specified by initialization parameters.
Set parameter LOG_CHECKPOINTS_TO_ALERT=TRUE to observe checkpoint start and end times in the database alert log.
Checkpoints can be forced with the ALTER SYSTEM CHECKPOINT; command.
