I am assuming you've read the previous articles on energy, elements, and the elemental triplicities. If you haven't, I recommend starting there and coming back.
The central metaphor of cancer is the crab with its shell. This is the domestic dimension of Cancer which builds a safe home base to return to. More generally, this aspect of Cancer energy seeks to make a safer environment to live in. What does safety mean? Mainly, it means a lack of negative surprises. This means treading the same path, and clearing it of obstacles to the best of your ability. The crab's shell is very thick and protects it against predators. However, the shell defines the full extent a crab may grow. The evolution of Cancer energy involves casting off the current shell as crabs do, being vulnerable for a time, and then growing a new shell to accomodate your more grown form. This might look like moving to a new place, getting a new job, or ending a relationship.
The root of the central metaphor is an emotional sensitivity. Cancer feels its feelings rather intensely, and it is this sensitivity that leads it to construct safe environments around itself. The comfort that comes with these environments is merely to lower the baseline negative stimuli. Cancer energy is incredibly important for healing precisely because it allows one to connect with their negative emotions and to properly grieve.
The drama triangle has a powerful grip on many people. Cancer energy may be pulled towards it due to the intense sensitivity. When someone appears to be a victim, Cancer may be tempted to leap into action and save the poor victim. However, Cancer energy often compells one towards a less intense relationship that is absent a persecutor. In this relationship, the one with Cancer energy acts to nurture, protect, and care for the other. The mental muscles of empathy, receptivity, and tender affirmation are strengthened by this activity, and it can enable one to be a powerful healer capable of helping others cope.
Dark Cancer energy is a disruption of the evolution process by holding onto a shell and not growing. Someone's career might begin with an entry-level job while they gain more experience to move to better-paying positions. Dark Cancer energy might fear that transition and hold onto the same job in fear of an unknown new job. Likewise, someone might hold onto a dysfunctional relationship for fear of the unknown of being alone or dating again.
The second shell can also turn dark. Healing allows someone with powerful empathy to feel positive emotions vicariously. Someone ruled by dark Cancer energy may use the positive emotions of the ones they heal in order to avoid their own emotions. This is the energy of one who focuses on volunteering and donating towards charity when their own needs are unmet. The sayings "you can not pour from an empty cup" and "give from your excess, not your reserve" hold true here. Cancer energy may compell some sense of charitability, but Dark Cancer energy compells self-sacrifice to an unhealthy degree. The negative beliefs tend to be "I cannot be ok unless you're ok" and "I don't deserve what I have".