Rebalancing a disk group moves data between disks to ensure that every file is evenly spread across all of the disks in a disk group. ASM automatically initiates a rebalance after storage configuration changes, such as when you add, drop, or resize disks. The power setting parameter determines the speed with which rebalancing operations occur.
ASM attempts to spread the data evenly across all disks in a disk group. It does this on a 'file' basis, and in the database, a 'datafile' assigned to a tablespace is a 'file'.
So when you add a new datafile, ASM tries to spread that file across all available disks in the diskgroup.
If you add another disk, or if you remove a disk (or it fails) ASM will automatically re-spread all files across the new disk layout for that group. This re-spread is called 'rebalance', which happens at disk add/remove time.
Normally a separate process is fired up to do that rebalance. This will take a certain amount of time. If you want it to happen faster, fire up more processes. You tell ASM it can add more processes by increasing the rebalance power.