Fast Backup is explained in detail in the help file. We also have an article that covers it and gives examples:
If the above does not help, then the following may.
What is a Fast Backup?
When you backup files to the destination it is assumed that no other application, or person, will be changing the files in the destination. (This also means 'no more than one Fast Backup profile', either in the same installation of SyncBackSE/Pro or another installation). For example, if you backup your files to another drive, and you are not going to be editing or changing those backup files (except using SyncBackSE/Pro to replace and delete them as appropriate, using a single Fast Backup profile on a single PC). Because of this, SyncBackSE/Pro is able to remember which files, and directories, are on the destination without needing to scan it every time to find out.
Using Fast Backup with slow destinations, e.g. FTP and networked drives, can considerably decrease the overall* backup time and load on your network.
The help file contains extensive information on Fast Backup.
How does Fast Backup work?
First, you need to enable the Fast Backup option for the profile. When the profile is next run, SyncBackSE/Pro will remember (by creating a database) which files and directories it copied to (or deleted from) the destination directory. This means the first run of a profile, after Fast Backup is enabled, will take the same amount of time as without Fast Backup enabled. However, for the second and subsequent runs of the profile (unless/until a Rescan is forced), it will not need to scan the destination directory because it remembers what it did the last time the profile was run. This means the scan time is substantially lower (at least twice as fast, often far more) especially if the destination is on a slow device, e.g. networked drive, FTP server, etc.
SyncBackSE/Pro V5 introduced Fast Backups that use the archive attribute instead of storing details of what the previous state of the files was. Whenever a file is created or modified, the archive attribute is automatically switched on (set) by Windows. With SyncBackSE/Pro you can create a Fast Backup profile that only copies files that have the archive attribute on/set. Once a file has been copied, its archive attribute is switched off (unset) by that profile. Any subsequent modification to the file means Windows will set the attribute again, and on the next run, SyncBackSE/Pro will detect that, re-copy the changed file, unset its attribute again, and so on. Special considerations apply using this mode - see the Help on the Fast Backup settings page for details. Bear in mind also that there is only one archive bit (flag) per file, so only one process can reference / change it, or conflicts will occur. That is, you can only have one process that is relying on this to detect 'copy required' state (and re-setting this flag after it has done so). If you have two or more processes, each will be resetting the archive bit that the other processes rely on to detect files that need copying (thus, any subsequent process will not see such files as 'candidates'). Each profile is a 'process' so you must only have one profile (and no other process from any other software) relying on and manipulating the archive bit (flag) of a given file. You can have multiple such bit-based profiles provided they do not reference the same files, but only one bit-based process of any kind that references any given file.
If you are doing a backup to an FTP server (and nothing and nobody else will be changing them independently on that side), it is advisable to use Fast Backup as not only will it reduce backup time but will also avoid problems with FTP servers that report the wrong file date & times, cannot set file date & times, etc.
With SyncBackPro V10, Fast Backup can be used with all cloud services (except Backblaze B2 unless used via the S3 compatibility interface).
*Note: Fast Backup has no effect on copying speed, but can radically reduce the scanning time, and it is often the case that the scanning time is the majority of the operation, if not many files were changed or are new. For example, a Fast Backup profile can potentially avoid scanning 10,000 files on an remote FTP Destination (very slowly) to find out only 10 files need copying there. It can thus radically reduce the overall time taken for the entire process, but it has no effect on the actual copy speed.