The way the 'light' of new understanding works today among JWs makes little sense, and has really become a loaded expression. The explanation of 'new light' from decades ago was more logical.
The Watchtower once explained by an illustration that it was like lighting up a building, that if there were 7 lights that it took to light a building, when the first light came on you could see so much of the bldg. When the second light came on, you could see more, and so on until all 7 lights lit the entire bldg.
The point was that 'new light' should always add to the light you had before, not abolish it. If you turned on a new light, but turned off a previous light at the same time, what have you gained? You still have the same amount of light as before, and have simply changed your beliefs.
Today we can see many beliefs that have been abandoned by the Watchtower in the name of gaining 'new light'. A good example is the ever-changing explanation of the 'generation that will not pass away.' Each new explanation effectively abolishes the previous one, rather than adding to it.
(Interesting, even the explanation of how 'new light' works has been changed. In todays Watchtower, claiming 'new light' is simply a way to avoid saying, 'We were wrong before.' It's loaded, loaded, loaded.)