Sit/stand bolt upright
Definition 1 of 1
Pro Tip 1/3Surprise Reaction
The phrase often indicates an immediate reaction to something unexpected or startling. 
Hearing the explosion, everyone in the room stood bolt upright.
Pro Tip 2/3Intensity
When used, it suggests a high level of intensity in the person's emotion, such as extreme alertness. 
The loud bang made me sit bolt upright, heart racing.
Pro Tip 3/3Physical Image
It creates a strong visual of a person's posture changing rapidly to a very straight, vertical position. 
In the presence of the general, the soldiers stood bolt upright.