Consider the example of a simple bedroom, where the bed is the focus. From the perspective of the bed, the back of the bed, its Black Tortoise side, is what is behind the bed head. In this simple example, the bed in Bedroom 1 is considered to have a solid backing. As for the bed in Bedroom 2, it is not considered to have a solid backing.  

Though the scale, shape and type of backing may varies, generally, 有靠山，就是有的靠。If you have a backing, it implies support. From who? The Black Tortoise side is representative of our parents, elders, superiors, literally those who have more resources than us, hence our patrons. With a backing, we can achieve more with less; 事半功倍. Without a solid backing, or even worse, if the Black Tortoise side is afflicted by Malefic Qi (煞气), well, then it is 每日无后靠，小人亲近我; susceptible to the negative influences of antagonists. An apple a day doesn't help in this case.

One may instinctively thought of the importance of Black Tortoise side for the salaried employee or for the vertical career progression in any hierarchical organization structure. Indeed. However, there is another crucial point for the Black Tortoise side, especially in many developed countries. And that is its influence regarding fertility rates and childbirth. But that is for another time.

Image of the Black Tortoise depiction from Beijing Ancient Observatory, by Vmenkov [CC BY 3.0], via Wikimedia Commons