This is my limited understanding:
If you are a normal body and you've run out of useable carbs and are tapping into fat reserves, you will be in trouble.
A "normal" body runs off glucose, which are readily available from carbs. To convert fats or protein into glucose is a taxing process.
If you are a ketosis-adapted body(this can take 1-10 weeks), your body does not run off of glucose. It runs off ketones.
Low-carb and Keto are not the same. Keto is basically no-carb, and instead of converting fat and protein into glucose, it will convert fat into ketones and the advantage here is that it highly prioritizes fat before protein.
I felt like a bag of smashed assholes for about 2 weeks, but now 6 weeks later, I feel like I have unlimited energy. My class looks at me weird when I start suiting up for kickboxing after BJJ class