Used to describe completing a task easily without feeling challenged. She is so smart that she didn't break a sweat during the exam.
Pro Tip 2/3
Sarcasm Indicator
May indicate sarcasm when someone pretends a task is easy but it's actually hard. Yeah right, like you didn't break a sweat putting together that huge puzzle.
Pro Tip 3/3
Underestimation
Might be used to show underestimation of the difficulty of an anticipated activity. He thought he wouldn't break a sweat fixing the roof, but it took all day.