Catecholamines and the fuel-mix shift
Caffeine blocks adenosine receptors, which among other effects increases circulating catecholamines such as adrenaline. Higher catecholamine signaling stimulates lipolysis, releasing more free fatty acids into the blood that working muscle can take up and burn. At moderate exercise intensities, this can tilt the balance of fuel use somewhat toward fat and away from carbohydrate, which is the substrate shift the diagram depicts. The effect is most apparent at lower to moderate intensities; at very high intensities, carbohydrate dominates regardless.
Caffeine's better-established performance benefits, including reduced perceived effort and improved endurance, may owe more to its effects on the central nervous system than to this fuel-mix change alone.
Why the fuel shift rarely equals fat loss
Burning a higher proportion of fat during a single session does not by itself reduce body fat over time. Total fat loss is governed by overall energy balance across days, and the body adjusts fuel use accordingly. Habitual caffeine users also tend to show blunted catecholamine responses, and individual sensitivity varies, so the substrate effect is neither large nor uniform.
This is general educational information about a mechanism, not medical or dosing advice. Caffeine affects heart rate, sleep, and anxiety, so consult a qualified professional before using it strategically, especially with a heart condition.