管用 (guǎnyòng) means 'effective, that works' — it's used for methods, medicines, or words that actually work in practice. It's more specific about practical results than 有用 (yǒuyòng 'useful') and more colloquial than 有效 (yǒuxiào 'effective, valid'). It differs from 灵 (líng 'efficacious'), which often implies a quick or even magical effect for remedies or predictions.