jerk (someone's) knee

Translations
провоцировать (кого-то) на резкую реакцию
His comment really jerked her knee and she reacted angrily.
Его комментарий действительно спровоцировал её, и она отреагировала с гневом.
Definitions
To cause someone to react quickly and often emotionally or without thinking, typically in response to a stimulus or event; derived from the physical 'knee-jerk' reflex, it refers to an automatic or reflexive response.