The 2 main types of beta receptors in the body are beta1 and beta2. Binding and activating beta1 receptors results in constriction of the blood vessels and an increased cardiac output. Activation of beta2 receptors (found mainly on the lungs) results in bronchodilation.
Clenbuterol is a beta2 agonist, and thus its normal effect is an opening of the airways. So you may be asking, why is that going to make me lose weight?
Well, clenbuterol is theoretically a good drug for asthma when administered at low doses. At low doses, the drug is only going to bind and activate beta2 receptors. However, when taken in higher doses, all beta2 receptors are filled up, and thus extra drug is left over to interact with and activate beta1 receptors.
Overall, this supplement is helping you to lose weight by forcing your heart to work far harder than it normally does, even while at a resting state.
Though it may seem like a great way to lose weight, I definitely wouldn't recommend the long term consequences of chemically stressing the heart and vasculature. Cutting a few extra pounds is not worth the risk of damaging the heart.