Translation Synchronization Capabilities
The Sync Mode (previously known as Merge Mode) saves a great deal of time for the editors, and for cases when two or more translators are working simultaneously on the same file, or when one has to maintain translations for several branches of software.
allows quick navigation through units that differ, and displays word-by-word differences. Also, has two Sync views - Primary Sync and Secondary Sync. They are identical, but the former is usually used to merge translations and second to keep in sync translations for two software branches.
After you copied translation from auxiliary file (synchronized it), any subsequent changes made to this unit will be replicated back to auxiliary file.

Merging
One use of Sync Mode is reviewing changes made by (new) contributors, when you cannot be sure of the quality of the work done.
Open a base file, then drop its changed version into the Primary Sync view, followed by Down or Up (remember that shortcuts may be modified in a usual way for all applications) to navigate through entries that are different.
used for merging changes in translation
used for merging changes in translation

Replication
Sync Mode may also be used to make changes to translation for two branches simultaneously. Set Branch folder path in your project options to the path that corresponds to base folder of the branch, and Secondary Sync view will automatically open files from branch. Then, each time you make changes in files of your main branch, they will automatically be replicated to the branch (of course, if it contains the same English string).
For example, if you work on KDE translation, you can checkout trunk to /home/xx/hacking/kde/trunk/l10n-kde4/YOUR_LANG and branch to /home/xx/hacking/kde/branches/stable/l10n-kde4/YOUR_LANG. Create project: /home/xx/hacking/kde/trunk/l10n-kde4/YOUR_LANG/project.lokalize and set BranchDir=../../../branches/stable/l10n-kde4/YOUR_LANG, then work via this project, and commit changes in both trunk and branch folders.

Alternate Translations
Each unit may have several alternate translations associated with it. Such translations may appear during file update, when the source string is slightly changed. In this case the old translation with it's (old) source is moved to alternate translations list, so that they are not lost.
When translating software, usually gettext tools are used to prepare translation files. When original text changes, gettext tools update translation files and mark entries with changed original text as fuzzy (or non-ready in other terminology). They store previous original text so that translators could see what changes exactly were made. simplifies life of translator and highlights parts of original text that were changed in Alternate Translations view.
highlighting parts of original text that were changed since translation was last reviewed
highlighting parts of original text that were changed since translation was last reviewed