Mac OS X Lion introduced MobileBackups, a new TimeMachine feature that stores local backups of your files while you are away from your main TimeMachine backup disk. This feature stores filesystem snapshots inside a virtual drive on your local disk, so should not be considered as a main backup strategy. The system automatically does periodic snapshots, but you can also manually initiate the process.
To make a local snapshot of the current state of your files, enter this command in your Terminal window:
sudo tmutil snapshot
It should complete almost instantaneously, giving you a tick in the TimeMachine backups timeline.