Entering the Tunnel Cart:

Spies enter a cart that is located in the tunnel.
There is a rope that enters the cart, and the guide explains that the spies must pull this rope to propel themselves forward.

Inside the Cart:

The spies must pull the ramp up and close the door to the cart. There is only space to stand.

Pulling the Rope:

The spies pull on the rope, thinking that they are propelling the cart forward.
The rope is actually attached to a pulley system that drives a conveyor belt above their heads.
There is an image of a sewer grate and a light that shines through it on the overhead conveyor belt.
The faster the spies pull, the faster the sewer grate images passes overhead again and again, creating the illusion of motion.
Hydraulic actuators underneath the cart and sound effects of rumbling convince the spies that they are moving in the tunnel towards the director's house.
 

Exiting the Cart:

The spies, tired from pulling, exit the cart through the side door which is also a ramp.
They are now outside the security door to the director's house and must hotwire it to proceed.