KanjiJLPT N1
Sudden

suddenly, immediately, in a hurry, arrange, stay in place, bow, kowtow

Mnemonic

A messenger page sprints into the barracks at midnight, slamming the door open. The sudden crash jolts every soldier awake, heads snapping up from their pillows. No warning, no gradual noise—just the page standing there, breathless, with orders that change everything in an instant.
Additional thoughts
Picture the sharp sound of the barracks door being thrown open—that jarring, immediate noise is the feeling of this kanji. The page's urgent arrival and every soldier's head jerking upward reinforces the bowing/head motion tied to the character.
Quick recall
A page bursts into the barracks with a sudden crash, snapping every head upright.

Details

The keyword for 頓 is sudden. This kanji conveys the idea of something happening abruptly, without warning or gradual transition. It carries connotations of immediacy and swiftness, suggesting a rapid change in state or circumstance. In classical usage, it can also imply bowing one's head down quickly, reinforcing the sense of a sharp, decisive movement.
On'yomi
とん、とつ
Kun'yomi
にわか.に、とん.と、つまず.く、とみ.に、ぬかずく