Copy only the clusters in files that have changed since the last backup.
For example, a 1GB file that has only changed by 1MB since the last backup only requires 1MB of storage in the incremental backup.
If a full backup is not present in the target folder then a full backup runs instead.
Incremental backups reduce backup time and image size. When you restore an incremental backup, you need the most recent full backup and every incremental backup between.