The check of synchronized data is triggered by the File -> Check synchronized menu or the ⌘K shortcut.

The archive parameter

The --archive parameter is the normal parameter to use in synchronize and snapshot tasks because it is fast. The --archive parameter to rsync preserves a lot of attributes of files when synchronizing. Files transferred in –archive mode ensures that symbolic links, devices, permissions, ownerships, modification times, ACLs, and extended attributes are preserved. When synchronizing files based upon the –archive parameter, rsync compares file size and last modification time to compute which files to be synchronized.

The checksum parameter

The --checksum parameters forces rsync to evaluate files based upon 128-bit MD5 checksum. Rsync computes and compares the checksum of all files. Files with not equal checksum are listed. This is a time consuming task because rsync computes and compare the checksum for every file and it is therefore best for verifying a backup.

About the check function
RsyncOSX does not evaluate the result from the check function. The user has to evaluate the result self and take required actions.

The ⌘K shortcut kicks off the --checksum process. The output is appended and refreshed every time rsync output a new line.

If the check finds files not synchronized a touch command on file will update timestamp on file and next synchronize task will copy missing files.