Debian Policy Manual
Appendix G - Diversions - overriding a package's version of a file (from old Packaging Manual)

It is possible to have dpkg not overwrite a file when it reinstalls the package it belongs to, and to have it put the file from the package somewhere else instead.

This can be used locally to override a package's version of a file, or by one package to override another's version (or provide a wrapper for it).

Before deciding to use a diversion, read Alternative versions of an interface - update-alternatives (from old Packaging Manual), Appendix F to see if you really want a diversion rather than several alternative versions of a program.

There is a diversion list, which is read by dpkg, and updated by a special program dpkg-divert. Please see dpkg-divert(8) for full details of its operation.

When a package wishes to divert a file from another, it should call dpkg-divert in its preinst to add the diversion and rename the existing file. For example, supposing that a smailwrapper package wishes to install a wrapper around /usr/sbin/smail:

       if [ install = "$1" -o upgrade = "$1" ]; then
          dpkg-divert --package smailwrapper --add --rename \
             --divert /usr/sbin/smail.real /usr/sbin/smail
       fi

Testing $1 is necessary so that the script doesn't try to add the diversion again when smailwrapper is upgraded. The --package smailwrapper ensures that smailwrapper's copy of /usr/sbin/smail can bypass the diversion and get installed as the true version.

The postrm has to do the reverse:

       if [ remove = "$1" ]; then
          dpkg-divert --package smailwrapper --remove --rename \
             --divert /usr/sbin/smail.real /usr/sbin/smail
       fi

Do not attempt to divert a file which is vitally important for the system's operation - when using dpkg-divert there is a time, after it has been diverted but before dpkg has installed the new version, when the file does not exist.

Debian Policy Manual