The first half of season eight of The Walking Dead began with Rick and Co. leading a horde of zombies to the Sanctuary. It was a crucial element of their strategy to defeat Negan: With hundreds of zombie’s surrounding Negan’s compound, they weren’t able to get out and supplies were not able to get in. Negan and the Saviors, in fact, were pinned inside the Sanctuary through the first seven episodes until Eugene devised a plan to clear the area of zombies and the Saviors could escape.
The plan was successful, too. Negan and his men escaped and commenced their destruction of Alexandria in the midseason finale. Negan gave Eugene big ups for his plan — which involved Eugene making bullets for the Saviors — but we never got to see exactly how The Saviors managed to clear the zombies. There’s a gap between the moment that Eugene comes up with a plan and the moment in which Rick and Jadis return to the Sanctuary to find that the Saviors had escaped. Meanwhile, Morgan — who was acting as a sharp-shooter for Rick — had also disappeared from his post, though we were left wondering what had happened to him, as well.
In this clip from the upcoming midseason finale (originally posted by IGN), The Walking Dead jumps slightly back in the timeline and fills in that gap.