The system uses a 2D LIDAR that can only detect distances in a plane. The spring is to give it a random pattern to ensure it scans in all directions more-or-less evenly. The readings from the LIDAR are then combined with the information from the accelerometer/gyroscope (can't remember which) to produce a 3D map of the environment in which the scan was taken. It's basically a LIDAR-on-a-spring-on-a-stick that you hold while walking around a cave.
Managed to see them demonstrate it when our robotics class went on a field trip to the Autonomous Systems Lab (along with a bunch of other cool stuff) and it was really as simple as walking around while holding it to get a 3D map of an area.