A low energy diet is really just a diet low in both carbohydrates and fat.
It is basically what you are left with if you eat low carb and low fat at the same time.

Low energy diets are highest in protein and fiber, and they contain fewer carbs and fats than the standard American diet.
The Average American is getting adequate protein, but they have to significantly overeat carbohydrate in the process.

The first step to eating a low energy diet is food choice, and choosing foods that have a higher protein percentage or higher fiber.
Avoid refined carbs like sugar and flour. Avoid refined fats like oil and butter.
Any type of non-starchy above-ground vegetable is excellent for a low energy diet.
Any high-quality, properly raised animal product is also a good component of a low energy diet. Leaner is better.
If you were to graph out the grams of protein and fiber in a food, a low energy diet will have higher grams of protein and fiber compared to grams of non-fiber carbohydrates and fat, as shown on the vector graph below.
The goal of a low energy diet is to avoid refined carbs and fats, and to ESPECIALLY avoid high energy density carbs and fats together.