Two-part allegorical saying

Sneeze your waist. The literal meaning of this two-part allegorical saying is easy to understand. Sneezing is a normal little thing, but flashing your waist is pure strength. In real life, this kind of two-part allegorical saying is mainly used to describe something that should not have happened, but happened unexpectedly. An unrelated thing happened unexpectedly, but it happened by coincidence.