gerama

Question

Perhaps I misunderstood, but after reading this website, I was under the impression that, as long as the effect was not immediate, I can change a mechanical Shabbat clock on Shabbat. From what I understood that action would make me a gerama, and is OK to be a gerama on Shabbat. Recently I was told that although true, it was forbidden by the rabbis. On the other hand, I have never heard that one should not use their refrigerator on Shabbat, yet opening it would be a gerama for the cooler to kick in eventually—I think. Was it forbidden to be a gerama by the rabbis? If yes, then how come we can use the refrigerator and change the Shabbat clock etc? Thanks, I am new to this and am trying to make sense of it.