Lara inside the Secret, with close view of MP's left arm. It's a warm up for Secret #6 of Lud's Gate (which is much more complex but quite similar).
Many trials show that the upper laser MP can close the gate of the secret just by running over it. Do a hidden jump in the pit, and run into the secret crawlspace without shooting the lower MP (since the upper MP would close the gate at your first bang-bang, he's very careful).
This secret is ultimately lost when Lara drops to the underground hallway.