The feature started to roll out on March 7. Once the update is official a user will be able to record and send a 30 sec video message if and when the recipient is not picking up the call or declines the call for some odd reason.
The video will show up for the person receiving the call next to the name of the contact who sent it. You can now leave video messages when somebody misses or declines your call on the duo. Additionally, if you do save your video messages for later, they'll back up to your Google Drive account, as well as to your phone.
Google says that Video Messages in Duo will begin rolling out to both iOS and Android users from today, but will take a few days to arrive on handsets globally. You can see how it all works in the video below.
To access video messages, just call someone over duo and as it's ringing, look for the new "Leave video message" button above the end-call button. Although, users will have the option to save any videos that they want to keep.