It's when you cover your entire ride in a tunnel to see if nothing's too close to the train. Put your entire ride in a tunnel, and when you ride it, see if anything other then track is in the tunnel. If it is, then your riders will be sure to lose some limbs.
There are some exceptions and things that are acceptable, but those are mainly tunnel-to-tunnel collisions on the same block segment or perhaps small things that are near the very bottom (like supports) and not of danger to riders!