The main active ingredient in thermogenic fat burners is caffeine. Caffeine itimulates thermogenesis (heat production).
The nutrients that enter your body are broken down and immediately used or stored. Energy production from breakdown is never complete, all energy 'leaks out' and this raises our internal temperature - this heat production is called thermogenesis. This is a very super process for fat loss, as it allows only part of the energy content of the food that enters the body to be used.
This process is stimulated by thermogenic fat burners, which allow the body to store less energy from food and more to raise the internal temperature. This heat, in turn, has to be dissipated by sweating, so thermogenic fat burners also make us sweat more.
Such termogenic fat burners can have a stimulant effect, such as caffeine (green tea and coffee bean extract) and ephedrine. Stimultans act on the adrenaline receptors, increasing blood pressure (as a stimultant) to keep you alert and awake - they can enhance performance during exercise. However, they are not recommended for people who are severely obese or have cardiovascular disease.