FAQ:What is 'bumping'?
From The Open Source Backup Wiki (Amanda, MySQL Backup, BackupPC)
The basic goal of "bumping" is to save precious space on the backup media as higher level incremental backups are smaller in size than lower level incremental backups.
The disadvantage of increasing backup levels is the fact that restoring from higher level incremental backups needs more tapes. This increases the amount of work time needed to fully restore a DLE as well as the possibility of tape-errors and similar problems during the process of restore. So in general it is recommended to keep the levels as low as possible with the given hardware and data.
There are various amanda.conf(5) parameters to control and fine-tune Amanda's behavior when it comes to bumping. Please refer to the amanda-manpage and the example amanda.conf for details on the parameters bumppercent, bumpsize, bumpdays and bumpmult.