Vanish Mode is opt-in only, therefore meaning that the recipient must also agree to activate the feature in order to send and receive messages through it. Messages are set to disappear automatically after it has been seen, with enough time for the sender and receiver to trade a few words before that. Once the session ends, both users are then redirected back to the original Messenger chat window. Additionally, all text and images are wiped clean after the mode is closed, leaving no trace of it in the chat history. Security and privacy-wise, Facebook says that you can report individuals over Vanish Mode chat conduct in the instance of abuse by the sender. Messenger will also notify the user if it detects that the recipient takes a screenshot of the chat when the mode is active. To use Vanish Mode, first access an existing chat thread in Messenger and then simply swipe up to activate it. Alternatively, you can also turn on the feature in the settings menu of a particular chat message. If you wish to end the session earlier, just swipe up again to return to the regular chat window.
The new Messenger feature has already rolled out to certain users at this time. A wider rollout is expected to arrive in the US first, with other countries to follow sometime later. With the recent integration of Messenger to Instagram, users on the latter platform will also have access to Vanish Mode once the update is available. Facebook has not revealed any plans to include this mode on the desktop or web version of its private messaging app. (Source: Facebook [Messenger News page] via XDA)
