The structure of the idiom is a direct and powerful contrast. It juxtaposes two scenarios: `当面` (in front of you) and `背后` (behind you). In each scenario, the person employs a different `一套` (a “set” of words or actions), exposing their duplicity.
This phrase touches on deep-seated cultural values in China related to social harmony, indirectness, and the concept of 面子 (miànzi), or “face.” In both Western and Chinese cultures, being two-faced is seen as a serious character flaw. However, the emphasis in Chinese culture on maintaining surface harmony (`和谐, héxié`) and avoiding direct confrontation can create complex social situations. While direct honesty is valued, openly criticizing someone can cause both parties to “lose face,” which is a major social taboo. A Westerner might say, “If you have a problem with me, say it to my face.” This direct approach is often seen as virtuous. In a Chinese context, while the backstabbing behavior of `当面一套，背后一套` is universally condemned, the impulse to avoid direct, face-to-face conflict is strong. This idiom thus serves as a powerful condemnation for those who take this indirectness to a malicious and hypocritical extreme. It separates socially graceful indirectness from outright deceitful behavior. It's the weaponization of indirectness for personal gain or to harm others, which is considered a violation of trust and social decency.
This is a very common phrase used to criticize people in various settings. It is almost always an accusation and carries a strong negative judgment.
The phrase is generally informal but can be used in more serious contexts to level a strong accusation. Because it's so condemning, you should be sure of the facts before using it to describe someone directly.