This is something I found a while ago, and figured I would share since I'm starting to want to get into speedrunning this game sometime soon.
When you get to the "C", you pull it down to the next level. Get on the right side, and start pushing it into the gap. As it falls, continue holding left. As the "C" falls and you continue to walk into the pit, you should land on top of the "C" briefly. After that, you can quickly walk to the right. If you did it correctly and fall out of bounds, you just keep walking to the right and jumping, eventually resetting the camera at the top of the stairs.
My best guess as to why it works is that when you land on top of the "C" you regain control and can change direction as opposed to just falling. Then, when you walk to the right during the transition, there isn't a wall to stop you from walking out of bounds.
