A harmless white smoke is forced through the sealed sewer system. A sealed system means that there are very specific locations that the smoke should be able to exit the system. When a system is fully sealed the smoke will exit at these locations and nowhere else.
When smoke exits the system at locations OTHER THAN the identified locations it represents a crack, leak, or damage to the pipe system. Mr. Rehab identifies the locations of the leaks, takes pictures, and logs the location so that repairs to the system can be completed at a later time.