Accelerator is a NetBackup feature which implements a "track log" on a per-drive basis to detect changed files. It allows for what amounts to full backups for the "cost" (in terms of CPU/RAM/network utilization etc.) of an incremental. Accelerator "fulls" are fairly analagous to what you normally think of with synthetic fulls. Additionally, incrementals typically take less time as well, especially on filesystems with large numbers of files, as unchanged files (for the most aprt) don't need to be enumerated.
"Accelerator forced rescan" is an option which forces NetBackup to enumerate through the filesystem and checksum all the files to ensure nothing has been missed somehow. For example, it's rare (but possible) for file data to change without the file metadata changing. Any files which have previously undetected changes, etc. will be refreshed during the scan. This option has less cost than a "full," but more than a typical "incremental" and should be used periodically for safety.
