To get all the weapons and loads of ammunition, follow the steps below:

Take one careful step forwards, one careful step backwards, turn around three times on the spot and jump backwards!

For a level skip, follow these steps:

Make Lara take one "careful" step forward, and then without releasing the walk key,(Which in most cases is the "SHIFT" key) take another "careful" step back. Then, make her turn around two full times, and make her jump forward.