你是他备胎我让他爆胎
[nĭ shì tā bèi tāi wŏ ràng tā bào tāi]
It literally translates to 'You are his spare tire, and I will let him have a blowout.' This implies if one is a backup option for a romantic interest, this person will make it problematic for that interest as a revengeful response to their behavior.