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 simplest notion of Sagittarius is that of the curiosity which compels one to explore the unknown. Suppose beans and rice are your favorite food. You are granted a lifetime supply of beans and rice and eat it for every meal with some vitamins so you will meet all your nutritional needs. The first couple days are great, and you're in heaven! After a few more days, the beans and rice start to get bland. After a month, you're craving something, anything other than beans and rice. You might cry out "beans and rice again!?" Replace "beans and rice" with the experiences you get in your life within your little corner of the world. This is the notion of Sagittarius energy. There are three horizons that one may expand, and they each have their own archetype.
This is the physical dimension of exploration. Experimental archaeology fits this, and so does taking a vacation abroad. Trying new types of food, new fashion, new music, sports, traditions/rituals are all good examples.
Have you found yourself reading random wikipedia articles for the fun of it? Do you look for new words? This is that. It's your appetite for trivia, your appetite for learning to speak in pig latin, or to identify all the Korean dynasties. This might also be the curiosity behind "how does X actually work?" This knowledge may come in handy in unexpected situations, such as as saying "this situation reminds me of the fourth Korean dynasty when such-and-such happened and they solved it with such-and-such solution".
The scholar asks "what?" and "how?", but the philosopher asks "why?". Knowledge might allow you to calculate the lethal dose of caffeine for your body weight and thus the highest non-lethal dose you can consume. Wisdom will tell you not to actually do that. The philosopher does not sit on one stable philosophy and extol its values. Instead, it is an appetite for new philosophies, new perspectives. I know my current understanding is X, but I also know other people see things differently. How do they see it, and why? That's the philosopher's realm. This often results in syncretism wherein one's wisdom pulls from an eclectic set of sources. The appetite itself is the energy, however.
That dark expression of Sagittarius energy is "old age", or the death of curiosity, or feeling contentment with what one already has and having no desire for it to change. It's "old man yelling at clouds" energy. The most egregious example of this can be found with the stereotypical boomer, who has a certain set of experiences, knowledge, and philosophies that they feel should be duplicated across the rest of humanity. They have the one and only true way, in their own eyes. It is not just different and uncomfortable for someone to be queer in their eyes, but actively harmful to the one true way. The refusal to believe food allergies exist is another example, as well as the assertion that culture has eroded as it has strayed from whatever cultural touchstones inspire the most nostalgia. Nostalgia itself can be a source for dark sagittarius energy. One might return to a prior aesthetic and explore it, but dark Sagittarius energy would contend that this subject of nostalgia is somehow superior to its alternatives.