Backup Agent is just a wrapper around the functionality that BUTIL -STARTBU uses. THey both do the same basic thing, just in different presentations.
If the open files are in "Contnuous Operations" mode (by STARTBU or BA), Delta files are created and any changes are logged to the Delta files. Once Continuous Ops is turned off, those changes are merged into the main file.
You really should take a look at the docs on COntinuous Operations. Here's a good start:
http://www.pervasive.com/library/doc...vops-09-5.html