Enter room C4 and walk forward until you reach a dead end. Enter the vent in the wall on your left. Ignore the first right and continue following the vent until you emerge in a room that has the hole in the center of it. Swing across the hole to the other side where a power puzzle awaits.
Your first step is to connect to the electrical outlet. Wrap around the two poles then look at the Green Hand outlet. Release the hand and the guard will drop allowing you to plugin the Green Hand. Grab the Pink VHS Tape (6) from inside then charge the Green Hand then swing back across the gap. Trigger the green outlet then run through the open door into the next room. Run back through the other door that opens so you can get back to the room with the poles. Wrap around them, look through the open door and finish the circuit. This video shows the entire sequence.
Once the circuit is completed you will need to go back through the vent. When you try to go to the door you opened you will fall down into the room below. In this room walk forward towards the end of the hallway with the hole in its roof. As you near, Mommy Long Legs will come down the hole. Turn and run away or else she will attack you.