There're several reasons why you want to enable the debug mode in disk utility. The primary of which is to be able to see and manipulate all of the partitions currently installed on your drives.  Here's how you do it:

1. Open up terminal and type the following: sudo defaults write com.apple.DiskUtility DUDebugMenuEnabled 1

2. Restart Disk Utility if its open.

3. You now how the debug drop down menu

Keep in mind that editing hidden partitions may not be a good idea in most situations. The GUID partition scheme that OS X uses always will have a hidden 200MB "EFI" partition as the first partition on the drive (s1), which is used to manage the drive and its partitions in systems with EFI firmware (which is what all Intel Macs use). Beyond this, the OS X 10.7 Recovery partition is also required for managing the new version of FileVault, so if you edit this partition on a FileVault-enabled volume then you may break the encryption keys stored on it and not be able to boot your system or read any data from it.