I didn't get it either, but I found it out with a walkthrough!

First you have to make sure you placed all the boxes on their switch and the door has opened. After that you remove the box on the switch in the middle, than remove the box on your left when Lara is looking at the door. Now you go to the switch of the last box you removed, and stand on it while looking at the chain that belongs to this switch. Wait till the chain is on a height that makes it able for Lara to go hang on it from the platform next to it. Climb higher and you will see an edge, you have to jump to it. This works best when you hang on the right height of the chain. After you hang on the edge, follow it to the right untill you can go higher (jump button). Follow the horizontal chain to the other side and let yourself fall to the edge on this side. Now you have to jump to the stick that's right of you, this works best when you go the final right point of the edge, hang a little bit to the right and then jump. Now you have to throw yourself to the other platform, and here you have a checkpoint again! When you go trough the door, you have to follow the steps they tell you (I believe you have to jump first and then roll...).