There are 2 mechanisms at work:
HCA blocks fat by incapacitating a key enzyme called citrate lyase. citrate lyase is responsible for converting carbohydrates or sugar that are not used immediately or stored in other forms into fat. The inhibiting of citrate lyase would halted the fat-making process and the production of triglycerides and bad cholesterol (LDL).
HCA works in a way that it eliminates hunger. HCA also works by suppressing appetite via increasing the levels of serotonin (Serotonin is a neurotransmitter in your brain that makes you feel good). Having low levels of serotonin may make a person feel depressed or anxious; and thus may drives many people into emotional or reactive eating. The increasing level of serotonin gives a person an increased sense of well-being and reduces emotional eating. When a person eats less, the body responds by releasing stored fat in cells.