The pressure pad that opens the secret door is on the ledge opposite the THOR SWITCH, shown in the screenshot above. Take a running jump across the gap to land there.
If you move to the edge and look down, you can see the secret door, indicated in this screenshot.
Until Lara steps off the pad, the timer won't start. So take all the time you want to get her into position: Turn so the central structure is on Lara's left and her feet are on the back edge of the pad (as shown above). Make sure her health bar is at least half full, since she'll lose some in the process, and save your game if you can.
Now comes the speed run: Hold the Action key and take two hops back to drop and hang from the edge.
Immediately let go and fall to the ledge below.
As Lara falls, press and hold Roll, so that when she hits the ledge, she rolls and comes up facing in the opposite direction.
Run forward and fall down to the next lower level. As Lara falls, again hold the Roll button so she lands and comes up facing the other way.
Run forward along the long ledge. If you want to, you can draw pistols and kill the 2 bats that swoop in from the left. If this distracts you, don't worry about it. You can get the bats later. Just keep running off the edge. . .
. . . and straight on through the timed door. (Don't jump off the ledge, since it will actually take longer to stand up and start running again than it would to just fall down and continue running.) Inside is secret #4.