Tea and some other plants contain antioxidative substances called catechins. Besides their antioxidative activity they share with other common substances, like e.g. vitamin E and C or selenium, catechins to possess other interesting properties regarding cognitive performance, dental health, and fat burning. There are indeed quite a few studies showing clear associations with (mostly green) tea catechins consumption and body composition. The exact mechanisms are yet unclear. But hypotheses involve the blocking of carbohydrate and fat digesting enzymes, inhibition of triglycerides (fat) transport into fat cells, and the induction of fat burning enzymes.
Studies have been designed with green tea consumption only or combined with other potential fat burning substances like e.g. caffeine, hot spices, and chlorogenic acid from green coffee extract. One study found that green tea has thermogenic properties and promotes fat oxidation beyond that explained by its caffeine content per se (1). Green tea extract may therefore play a role in the control of body composition via activation of thermogenesis (heat production by burning calories) and fat oxidation. This conclusion based on the measured total 24 h energy expenditure and the so-called respiratory quotient, which allows an estimation of calories burnt from fat. The subjects received green tea extract (90 mg catechins and 50 mg caffeine), caffeine only, or placebo at breakfast, lunch, and dinner. Results showed a highly significant increase in total energy expenditure (p < 0.01, i.e. probability of a coincidental result less than 1%) and an even more significant change in the respiratory quotient (p < 0.001, meaning 99.9% confidence level). This means that a bigger part of total energy was burnt and thereof more fat-derived calories compared to placebo.
Sponser offers a product called LOW CARB BURNER containing 90 mg green tea catechins per drink. It additionally delivers green coffee extract, peptides, and fibres. Caffeine has not been added by purpose, since some people do not want caffeine because of its rousing effect. But everybody looking for the combined effect of green tea and caffeine on fat metabolism takes the ready-to-drink ACTIVATOR ampoule to dose its caffeine at wish or boost up his training with the whole serving of 160 mg caffeine.

Sources:

(1) Dulloo AG, Duret C, Rohrer D, Girardier L, Mensi N, Fathi M, Chantre P, Vandermander J. Efficacy of a green tea extract rich in catechin polyphenols and caffeine in increasing 24-h energy expenditure and fat oxidation in humans. Am J Clin Nutr 1999; 70: 1040-1045.