Beef pounded rather than minced into a smooth paste, shaped into balls and boiled, which makes them springy. Already cooked, they only need heating through in hot pot or noodle soup.
A few beef meatballs in the hot pot make it heartier.
Tips
culture
The famous version is 潮汕牛肉丸 (Chaoshan-style) from eastern Guangdong - made by pounding the beef with heavy iron bars until a paste forms, giving the balls a famously springy bite (Q弹). Made a cameo appearance in Stephen Chow's film 《食神》 as the fictional 'exploding' 撒尿牛丸.