So how does “open-hang” come to mean cheating? The character 挂 (guà) here is shorthand for 外挂 (wàiguà), which means “external plug-in” or “add-on software”—the kind used for cheating in games. Therefore, 开挂 (kāi guà) is short for 开启外挂 (kāiqǐ wàiguà), which literally means “to activate an external plug-in/cheat.” This origin is key to understanding its modern metaphorical meaning of having some kind of secret, external power source for success.
开挂 is highly informal and is a cornerstone of modern internet and youth slang. Its connotation changes dramatically with context.