When starting up the MAC holding down the option key, it will bring you to a menu to let you choose boot from the Recovery HD.
When install the Lion upgrade the upgrade will create a small 650mb bootable disk that isn’t part of the main partition.
The menu will allow you to reinstall Lion, restore from a Time Machine Backup, run Disk Utility or open a Safari to look for solution to any issues.
To download the installation dmg file.
1. Open the Mac App Store application.
2. Navigate to your Purchases page.
3. Hold down the Option key and click on the OS X Lion icon on the left hand side of the screen. Do not click on the Installed button.
4. You will be taken to the Lion product page. Hold down the Option key and click on the Installed button. If you haven’t already authenticated to the App Store you will be prompted for your login credentials, otherwise the download will start.
Once completed, you can then save the Installer DMG to another disk or burn a DVD.