Pick up the Vintage Rifle. There are about 7 or 8 little stone heads around the walls.
They have "nice faces" most of the time. Periodically, one of them will rotate in the wall
and it will be an "evil face" (snarling with red eyes). Shoot it! It will attempt to shoot
you with a little blue laser. If you hit it before it fires, great. If not - after it's
fired you still have a moment to shoot it while it rotates.

When you've shot the first one (which can be any of them) it will say "X heads remaining"
or "X faces remaining". When you have shot a face it is broken. You can see where it was,
but the face itself it smashed up.

ORDER OR PLAY
# Grab Vintage Rifle
# Find point where you can see as many heads as possible.
# When you see a head rotate, shoot it!
# When all faces are destroyed advance across the bridge.
# An aztec warrior will appear behind you as one appears in front of you.

Check a walkthrough if you want any more help.