His performance includes some of the following tricks: He swallows sugar, followed by a glass of water, and then regurgitates the sugar, completely dry. He swallows a live goldfish and regurgitates that a minute later, still living. (As he does this, he mentions the urban legend about goldfish having 5-second memories.) Reportedly he's also able to swallow a (miniature) rubik's cube and bring it back up — solved. (Though the Rubik's cube trick isn't shown in the google video.)
I can't find anyone on the web who has a decent explanation for how Starr is able to do all this. Obviously he has a genuine talent with his stomach. An article about him in the Amherst Student reports that:
he was born in a children’s home in Scotland, where he lived for the first 19 years of his life. When little Stevie was four years old, he discovered this unique talent by swallowing his lunch money and realizing he could bring it right back up. Thus, a freak of nature was born.
But this doesn't explain how he can swallow sugar, followed by water, and bring the sugar up dry. Or the trick with the rubik's cube. Does he have a second stomach, or something like that? To do the rubik's cube trick I assume he must have swallowed a solved rubik's cube before the show. But like I said, I'm pretty much baffled.
Incidentally, history is full of famous vomiters, so Stevie Starr evidently isn't the only one who has ever had this talent. In 1621 there was the case of the nail-vomiting Boy of Bilston (who had been trained by a priest to simulate the symptoms of being bewitched). This was followed in 1642 by Catharina Geisslerin, "the toad-vomiting woman of Germany," who, as you might guess, had a talent for vomiting up toads. In 1694 there was Theodorus Döderlein, who vomited up twenty-one newts and four frogs. (I'm getting this info from Clifford Pickover's The Girl Who Gave Birth to Rabbits.) Pickover also reports that there have been cases of compulsive swallowers who don't later regurgitate what they swallow, including one guy in 1985 who had "53 toothbrushes, 2 razors, 2 telescopic aerials, and 150 handles of disposable razors" removed from his stomach.