To have a knee jerk reaction means to react automatically. When somebody or something knocks your knee, you instantly jerk it back. Similarly, when somebody says or does something you could respond automatically and quickly without a thought for the consequences.
1. The formation of the Telangana State was a knee-jerk reaction by the Govt. to the volatile situation in Andhra Pradesh.
2. The RBI measures were a knee-jerk reaction to the rising rates.
3. The plumber pushed his finger into the pipe's hole when the water came gushing out; it was a knee-jerk reaction and he did not realize that his finger would get stuck.