This is one of two alternate endings. Spoilers.
*** Spoiler - click to reveal ***After turning the massive light bulb green, head straight up util you reach a room. Place your friend on the platform to the upper-right. The big door will open. Fly to safety and leave your friend behind. The achievement pops once you're high enough up outside.
Once completing one ending, pick Extended>Continue from the main menu to simply replay the very last part described. Thanks MC0REBE for the reminder of that.
This video from "aka Scratch" demonstrates this achievement. Same spoilers within this as the text. Note that this videos shows both endings, Self-sacrifice first and Rest in Peace as from about 3:07.