Something takes hold of a body and it about, and the body cannot stop it. Picture shoulders jerking with sobs, or a leg jumping on its own after a long run: the movement is hard, sudden, and nobody chose it. Every use of convulse keeps those two parts — violent shaking, and no control. What changes is what is doing the pulling (a muscle, a feeling, a crisis) and how big the body is (a person, a crowd, a whole country).
From the word’s history
Wiktionary traces convulse to Latin convellere, from com- (together, thoroughly) plus vellere (to pluck, to pull). The literal sense is to pull hard at something, which is exactly the picture the modern word keeps.
來源資料把這個字追到拉丁文 convellere：com-（徹底、一起）加上 vellere（拔、猛拉），字面就是「用力扯動」。這條線索跟現代的畫面完全對得上——不管扯的是肌肉、情緒還是整個社會，被扯的一方都只能跟著抖。
Wiktionary entry for convulse; packet etymology_text ("From Latin convulsus, past participle of convellere ... from com- + vellere"), source_ref kaikki:3f6f69a190036ae4:767107101:9575
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