The use of CBT (Changed Block Tracking) for incremental backups will identify and track blocks, which have changed since the last backup. These changes are logged in database files and used by the next backup job.
The logged changes are read immediately as they happen, thereby completely eliminating the need for scanning for changes when the backup starts; hence the backup window time is greatly reduced.
Infrastructures running VMware ESXi will leverage VMware's own changed block tracking mechanism.
Infrastructures running Hyper-V will utilise our own implementation of changed block tracking, which was introduced in version 6.4. There are a number of conditions that must be met and that will trigger a scan for changes; these can be seen here.
You can check the reason for changed block tracking being skipped in the Operation History, as per the screenshot below: