This is a comprehensive guide to get a secret achievement.
Contest Propellant Achievement Guide
Getting the Achievement
Start by loading the chapter ”interloper” and progressing trough it like you normaly would until you reach this part.
After that press the button and be prepared to take out the mind controlers (i recomend you do this without killing a single vortigaunt if you want to do 100% as, you’ll need it for the ”PHD in pacifism” achievement). After that go trought this vent like you normally would.
Then look to your left, you should see two boxes. Get them out of the way and go up the stairs into another room and the achievement is done.