Dear Magdalena,
Note that the live comparison is of the currently-selected code in the left-hand side pane, when looking at an item's Citation details tab.
If you select an item and press Go to view its details, then select the Coding record tab, you can click the Live comparison button...
Then go back to the Citation details tab, and select a code from the left-hand pane...
The Live comparison details for that code will be shown at the bottom of the right-hand pane. You can step through items using the Previous and Next Item buttons towards the top-left of the pane.
(Note that the live comparison will only show coding information for the selected code "level". If you click on a codeset, you will only see coding information for the first level of coding below that codeset; similarly clicking on nested codes will only show the coding information for the first level of child codes underneath.)
Hope that makes sense, but do let us know if you require any further information. (There is some more detailed information on page 66 of the manual, available from https://eppi.ioe.ac.uk/cms/Default.aspx?tabid=2933.)
Kind regards,
Zak