The answer is quite simple:
Indy's hand cast a shadow on the wall... which in turn projected (through a series of hidden apertures, convex mirrors, and prisms) the silhouette of an enormous hand into a secret chamber full of bats and piranhas.
These creatures, thriving for centuries in this room, are rarely ever disturbed. Upon seeing the gigantic shadowy appendage, they flee in all directions, and one of them manages to trip a thin rope...
This rope pulls at a network of levers, weights and pulleys (the goonies anyone?) and VOILA! -a spiked trap is sprung!
How, you ask, did the trap manage to reset itself? Not once, but twice?
... the bats and piranhas get paid $6.65 an hour for tunnel maintenance.