[Users] When does oVirt auto-migrate, and what does HA do?

In what scenarios does oVirt auto-migrate VMs? I'm aware that it currently migates VMs when putting a host into maintenance, or when manually selecting migration via the web interface, but when else will hosts be migrated? Is there any automatic compensation for resource imbalances between hosts? I could find no documentation on this subject, if I missed it I apologize! In a related question, exactly what does enabling HA (Highly Available) mode do? The only documentation I could find on this is at http://www.ovirt.org/OVirt_3.0_Feature_Guide#High_availability but it is a bit vague, and being from 3.0, possibly out of date. Can someone briefly describe the HA migration algorithm? Thanks, Rob