This four-character idiom comes from a historical story about Qin Hui of the Song Dynasty: his conspiracy was exposed after being discussed under the east window. You can use it when a secret plot, wrongdoing, or hidden scheme is finally discovered and made public. It carries a strong negative connotation — the exposure leads to consequences (punishment, shame, failure). Unlike the English 'come to light', the Chinese phrase always implies someone’s deliberate malice or illegality.