First
You can take this shortcut when you first reach the area with the structure and slope with three holes.

Once you kill the mutant and pick up the Shotgun Shells from the alcove on the left, do not proceed to the right of the structure. Stay on the left and walk on the ledge that is attached on the structure. Take a running jump to grab the rocks on the left and shimmy to the right (left hand image). You need to shimmy until the spot where the highest hole of the slope is. Then pull up and presss Jump to back-flip on the slope, slide a bit and fall through the hole (right hand image). Then follow the walkthrough at the end, where it says Bottom of the pyramid.

Second
You can take this shortcut once you fall through the opening of the slope with the three holes and first enter the pyramid.

You need to fall through the left hand opening (image #1). Approach the opening and take a look below. The brownish ledge (pointed with the arrow in the image #2) is where you need to land. To do that, face east and take some steps backwards until Lara's back touches the wall (image #3). Now start running pressing Action at the same time. This will lower Lara's arc and make her land on the brownish ledge. When you do that, fall through the opening you will find there. Then follow the walkthrough at the end, where it says Bottom of the pyramid.