The rolling boulder sequence is also included in the gameplay video.
After the second blade trap, continue past CHECKPOINT 14, up a flight of stairs, around a corner and down another flight of stairs. Just before the next spear trap, a cut scene interrupts. Lara feels a rumbling as a huge boulder rolls toward her from behind.
If you go too slowly, or fall into the pit, Lara dies. Once you reach the doorway, another brief cut scene shows how she escapes. Proceed to the exit and CHECKPOINT 15.
NOTE: Earlier versions of this walkthrough included a different method for evading the rolling boulder. I mention it here in case the simpler technique above doesn't work for you. The camera doesn't respond to mouse control here so, if you're playing on the PC, take your hand off the mouse and use only the direction keys to steer Lara. You may want to use two hands on the keyboard—the right hand to move Lara using the Up, Left, and Right arrow keys and the left hand to roll and jump. (This is a lot easier using a controller with analog sticks.) As soon as the cut scene ends, run forward and a little to the right. Then jump to clear the first set of spears. Run forward, veering a little to the left. Then roll under the second set of spears. Run straight ahead and jump to clear the spike pit. (Thanks to Boscobuddy and Germano for these tips.)