We perform this test on the under-slab sewer pipes wherein we fill them with water. We observe if the water level drops and if there is a place for water to escape. Once we rule out all human error possibilities and the water level still drops, we can verify that there is indeed a leak in the pipe, with a test like this you are testing the entire under-slab/home sewer system you cannot determine if there is one leak or more than one leak, only that there is at least one.