If a movement is convulsive, it is sudden, strong, and the person cannot stop it. The body jerks or shakes hard, the way it does in a fit. Doctors use the word for that kind of shaking. Writers also use it for other bursts you cannot hold back: convulsive laughter, convulsive sobbing — the laugh or the cry takes over. It sounds a little like a doctor-word, and a little formal. In easy talk, people more often say a sudden jerk, or she could not stop shaking. Though actually, it is not only for a medical fit: a convulsive gulp, a convulsive shake of the shoulders — any movement that hits hard and will not be controlled.

痙攣的、抽搐的（也可形容笑、哭等猛然控制不住）