This is a super useful tool for saving you a lot of time as well as making it easy to have multiple standard outfits and configure body customization for those outfits. To use this tool you must be at a location where you can rez objects. This is easily confirmed by right clicking on the ground in world and seeing if you can create a prim. If not, you need to be somewhere else. Once you are certain you can rez click the "Backup Body Settings" button and the HUD will do the rest.
A body backup object will be rezzed over your head and once the backup has completed it will prompt you to enter a name for the backup. This way you can give the object a name that will help you remember what was special about the backup. I like to use names that go with the skin I'm using as a start.
Once you have given the backup a name then right click it and Take it into your inventory.
I like to stash them in a folder and then copy them and paste a link into an outfit folder. That way when I switch outfits, the backup is there on screen ready for me to click to switch the body to match the outfit. Then I can detach it and off I go :)
To use the backup later on you ADD it and it will appear on your HUD near the upper right corner of your display. After it appears, click on it and it will give a dialog with some helpful information and a few choices on what to do: Restore the body from the backup, or Update the backup to match your current body settings. Easy!
If you want to change it's name you MUST rez it to the ground as SecondLife does not allow objects that are worn to change their names.