Migration occurs two ways: a threshold is hit because client data is filling 
the pool, or the migration threshold is manually lowered forcing migration to 
occur.  Most systems use a little of both: i.e. there is a threshold set and 
sometime after the backup window an administrative schedule is run lowering the 
threshold to force the rest of the data to be migrated.  You can also choose to 
keep migrated data in the disk pool.  This is called caching and is set on the 
disk storage pool that ultimately migrates to tape.

If you do a query stgpool at the command line, and caching is turned on, you 
will see that the space utilized percentage rarely drops very low.  However, 
the percent migratable will drop to zero after migration is complete.  The 
percentage space utilized is the amount of cached data.

Kelly