Large Forerunner installations like Halos and large ships have Translocation Grids. They allow you to access Slipspace without a Slipspace drive. In the original trilogy, it was marked by a user being [url=http://youtu.be/fGTmrnJ4wbA?t=29s]encompassed[/url] by [url=http://youtu.be/XxRkdYaldXg?t=4m18s]yellow rings.[/url] Halo 4 features quite a few variations of the Translocation Grid. Requiem features a [url=http://www.halopedia.org/images/e/e4/H4-Forerunner-SlipspacePortal.jpg]static portal network[/url] and a dynamic portal network (You see this used individually by Sentinels or on a larger scale by Watchers). The Mantle's Approach also seems to feature more of a [url=http://youtu.be/EmR-Uuo96fY?t=2m3s]traditional Translocation Grid[/url] along with the other variants. Now for the explanation. On the last time you insert Cortana, she says something peculiar that she had never said before: "Quick! Merge me into the vessel!" This explains why she survives when the Didact detonates the plinth. She wasn't being stored locally, she integrated into the entire ship's mainframe. As Chief moves up to confront the Didact, you see him use the translocation grid to get around Chief. Fast forward to when he's going to detonate the nuke. Now, because Cortana was merged with the Approach, she had access to all its systems. Meaning Cortana translocated Chief off the ship, and also used the Approach to generate a Hardlight environment along with Artificial Gravity. Additionally, we know for a fact Cortana can translocate Chief off the ship because Forerunner translocation grids always extend outside their intended installation as shown in Halo 2 when the Gravemind uses Delta Halo's grid to translocate Chief to High Charity. Any questions?