in a few words: it feels like you are getting a handjob from the gods. I will try to describe the IV speedball rush as best as I can, but these things are hard to describe to somebody who has not indulged. imagine explaining to someone the taste of chocolate when they have never even tasted it before?
anyways, when you spike your vein you feel an initial cocaine rush. you feel instant coke euphoria, but then you still feel the warm taste of heroin in the back of your throat. instantly, you are geeked out from the stimulant and your heart is racing. you are geeked out, but in the back of your high is still a feeling of opiate bliss. then, as soon as the cocaine rush withers away you are in full heroin land, but still having the after-effects of a fat cocaine shot. at this point you are what I call "twacked out," neither experiencing a downer or an upper, but some debauched version of the two. speedballs are fun but it makes the next days w/ding pure hell.