(art reference)
More often than not, she’s a mercenary or high-ranking underground criminal.
No tragic backstory, she learned about her ability in a stressful situation where she needed to deliver precise, quick jabs. She found that if she held completely still for a moment, she could instantly release a jab. From there, it was experimentation to find out what the limit of her ability was.
She of course, used this ability primarily for combat. Doing protection jobs and assassinations.