How do local TimeMachine backups work exactly? Since Mac OS X 10.7 (Lion) TimeMachine performs local backups to /.MobileBackups if enabled (tmutil enableLocal, default enabled on portable macs).

The different types of backups are color encoded in the fancy TimeMachine space thingy, local backups are white, external storage purple. They mostly differ only by a minute. What exactly is the concept of those local backups? 

Are they simple a completely parallel, additional backup? Or are they at least merged into your external storage backup when you come home after begin away from your backup drive for a week? And what happens if i require lots of disk space suddenly? Do i need to remove the local backups manually?
I also thought that the Local Snapshots were supposed to merge into your Time Machine backups but it seems, in my experience, that they don't. You can see both when you're in Time Machine and "About This Mac" shows that they still exist after multiple backups to an external harddrive. I do think that they are supposed to merge with themselves the way that Time Machine backups do, to save space, merging into daily and weekly backups instead. As far as Finder is concerned, the space is not used. Looking in the info bar at the bottom of a Finder window showing your root drive will show that you have more space than something like iStat Nano will show on Dashboard. I think this is because the backups are supposed to be expendable. If your computer really needs the space I assume that it will get rid of some of its snapshots to make room.
We may see that in the future they merge back into regular Time Machine backups as Lion matures more.
It's also worth noting that I don't think they count as real 'backups' in the sense that you can't restore to them the same way you can restore to a time machine backup from the Recovery HD. They are really meant to help you recover a file instead of your whole filesystem.