Synephrine is used to activate the adrenaline system without the stimulatory effect posed by ephedra-based products.
Synephrine is a stimulant, similar to caffeine and ephedrine. It is thought to have similar effects in terms of providing an energy boost, suppressing appetite and increasing metabolic rate leading to the burning or more calories. It is used as a safer alternative to Ma Huang, synephrine is theorized to stimulate fat metabolism without the negative cardiovascular side effects experienced by some people with Ma Huang.
The most likely explanation for weight loss effects attributed to citrus aurantium (synephrine) supplements is the stimulant like effects of the alkaloids. Although this effect is likely to be somewhat less dramatic that effects induced by Ma Huang (ephedra - also known as Sida Cordifolia), users can expect variable effects including reduced appetite and heightened feelings of energy (similar to caffeine)... both of which are likely to result in weight loss.