For example :

a book has 3 authors : Victor Hugo, John Travolta, Nicolas Sarkozy. If the user click on one of the 3 authors, a small popup opens, with the 3 authors, the one he has clicked on is selected, and the user can check the other ones if he want, and launch the search on 1, 2, or the 3 authors.

For subject search, the idea is the same, except the user can select at subfield level.

For example, if a biblio has 3 subject : sports, Europe – France, Soccer –- FIFA world cup 2010, if the user click on one of them, 5 choices can be checked : Sports / Europe / France / Soccer / FIFA world cup 2010

for an example, see here http://cat-bib.nimes.fr/cgi-bin/koha/opac-detail.pl?biblionumber=9