When Loid tells you to hack the computer to transfer into the Vessel in the Sanctum Anatomica near the start of the Whisper in the Walls quest, use transference (default 5 key) to pop out into your operator mode and your warframe will be transported to the 1999 segment. It's funny because your warframe still has an HP bar in there so Arthur is effectively treated like a skin for your operator. Your warframe will also be present in the final cutscene with Albrecht. It works better with frames that are thinner/have less decorations because of the camera positioning and the clipping of the AX-52 through the larger warframe model. So it'll work nicely on say, Ember. But it won't look as good if you use Grendel (but it will be funny)
