To unlock this achievement, you need to deactivate the 12th orb. To find it quickly, load the 50th checkpoint. Go right down in the hole and keep going right until you reach the room with the yellow box. Now, you need to bring back up the yellow box to the testing room. Then, push the yellow box to the right side of the testing room (where you can see a unreachable red button). Use the yellow box to push the red button, a secret elevator will go down. Use again the yellow box to push the red button and jump on the moving elevator (on it's way up to the room with the orb). Watch the video below:
Room For Reflection achievement:Full walkthrough of the game: