I imagine that their emetic properties is closely linked to their dopamine agonist activity, as dopamine in the CTZ (chemoreceptor trigger zone) is a pretty good way of ensuring a bout of doing the big spit (I really like Hunter S Thompson's name for vomiting!). It's the dopamine agonist activity of apomorphine and emetine that makes them so good at instigating vomiting (calling an alkaloid emetine really makes you want to try it, doesn't it!)