The hidden tab asks for the PIN, and it explains how to set one when none is saved. Unlock lasts for the session. You can also relock the list after a number of minutes instead of leaving it open until the app closes.
An optional decoy PIN opens an empty hidden list and an empty vault. Someone who knows you have a PIN, and who is given the decoy, sees nothing to open. The safety PIN is the only one that shows the chats and the files.