Lactic acid is a by-product when the muscles use carbohydrates to create energy for exercise. Lactic acid then breaks down into hydrogen ions and lactate, and it's the hydrogen ions that cause the pain.
A great way to burn belly fat fast is with a lactic acid workout. Lactic training is a form of exercise were you train with the sole purpose of getting your body to produce lactic acid. This is done with fairly intense and short burst exercises, such as squats with very small rest periods.