This a basic fundamental move that often can be done when rolling. I noticed a lot of white belts when on top in half guard forget to under hook the far side opponents arm. When this happens the bottom guy can easily escape to the back as shown in the video.
Great explanation by a Lloyd Irvin Black Belt, Danny Ives, showing a simple way to get the full guard. This is a good one for beginners as this position is common when starting out doing BJJ.