Xiehouyu (or two-part allegorical saying) is a prevalent and unique way of expression in Chinese. Past studies have focused on classifications and sources of xiehouyu, touching barely on their emergence, usage or cognitive functions. Based on corpus survey and questionnaire reports, this study probes into the omissions, intentions of use, and effects of xiehouyu, and tries to apply theories of metaphor and metonymy to account for their working mechanism and structural properties. It is found that a) xiehouyu differs in terms of their familiarity, and there are idiomatic xiehouyu and novel xiehouyu, with the latter often omitting the second part, which is actually the most important part and whose meaning normally cannot be inferred from context; b) the omitted part is, in most cases, the ground of the metaphor or the reference point of the metonymy in question, not the target domain, as suggested by some scholars; c) more than 50% of those surveyed associate the use of xiehouyu with such personal traits as "humorous" and "clever", etc. The author thus claims that metaphors and metonymies in xiehouyu differ from others in that they are used not for cognitive purpose, but chiefly for social and interpersonal reasons.