Add key for ended giveaway, what does it do?

it will send the GA automatically.
the winner still needs to mark as received. but now SG moderators know that you really gave the key...

You might want to read the welcome note by cg http://www.steamgifts.com/discussion/ASyvq/were-alive.

"Keys

Giveaways now give you the option to attach a key. When creating the giveaway, just check the box for keys, and then input them into the text field, separated by new lines. You can enter keys up to 32 characters, and both Steam and Humble Bundle gift urls. You still have a chance to review your winners before the key is visible to them. When your giveaway closes, and you're viewing the winner, the sent checkbox will toggle the visibility of the key. When you have that box checked, the winner will be able to see either the key or gift link. If you uncheck the box, it'll be hidden again. You can add, edit, and delete keys for any winners after the giveaway is over.

The only exception are key giveaways for 50 copies or greater. These giveaways will automatically check the sent box once the giveaway ends. This is in place for general convenience and developer giveaways, so you don't have to manually check each box to distribute the keys."

How does the email look like? I'm curious. :D

When you enter a key, and check the send key box, the key will be visible on the winners page of won giveaways. If you uncheck the box, it'll be hidden again. You can edit the key at any time, and if you delete the key, it'll be completely removed from the database.

This still does not explain the "add key" function after the GA has ended to me. Will it add another winner, or just give the original winner a new key to try (if the first didn't work)?