Write HOSTS is basically just an export, but to the system's HOSTS file as selected in preferences. You are not shown a dialog - it just does it, showing an hourglass cursor for as long as it takes (which shouldn't be long).
The same options you used during your last export are used, but when the HOSTS file is exported, Good Hosts are always included and the default group selection is used. The default groups are all groups except those marked as eXcluded from the HOSTS file.
When the HOSTS file is disabled, the renamed copy of the HOSTS file is written to. None of the changes from the update will take effect until the HOSTS file is re-enabled.