For example you have a button with two status are normal and hover states, each state has a corresponding background. Normally you will save two files normal.gif (16x16px) and hover.gif (16x16px).
Now create a file using PS picture.gif (32x16px) then copy paste to 2 other images, the image on the left to normal, the right to hover image, like puzzle games so that.
Set CSS with code
When you use CSS spirits you have to keep some points in mind such as CSS files are little bit heavier, Give attention when you are using repeat. For using the CSS spirits you have to put multiple files together in one file.