Slide down the slope facing uphill. Near the end, press and hold Action so Lara grabs the end of the slide.
Pull up, wait a moment for Lara to slip off the edge and then press and hold the Jump button. Do not press Jump before she falls off the upper slope or she will jump from there and miss the opening to the secret.
If you hold Jump as she falls onto the short slope below, she'll land there. . .
. . . and immediately backflip to into the opening of the secret room.
Here Lara is standing inside the secret area after landing the jump.