Non spoiler version: This unlocks after you've collected enough storage devices from the D levels and the E levels are unlocked.
*** Spoiler - click to reveal ***Spoiler version: After Ellie sabotages the AI, you'll chase her through several levels. There's nothing to pick up here and you can't go to any other levels in the meanwhile.
For the most part, Ellie shows you how to complete the levels first. The only exception is the last level, involving a laser, floating box, and two very long sets of saws. Have the box flat against the wall with the laser to block it, then move as close as possible to the circular saws before flipping gravity. The idea is to not touch the box so it won't push you into the saws.