How Do you do a tunnel test

I read on nolimits sites about tunnel test and I have no idea what that means or how you do them. How do you?

When designing coasters, "real" engineers must factor in a safety envelope around the track to make sure that riders won't get injured if they stick out their hands, lean out of the car, etc. In No Limits, you can make sure that your track has proper clearance by adding a tunnel to the entire ride. If the tunnel isn't intersected by supports, other track segments, theming, and so on, then your ride safe. *** Edited 7/21/2008 3:28:38 AM UTC by PhantomTails***

Thanks Phantom that makes sense. I see it's exactly like when they put a wooden board on the front of the coaster and pull the train through the course. It is used to make so riders have proper clearance when riding the ride.