When the pressure placed on a button is just before the breaking point. If there is a thread failure, when the button comes off, the noise it makes is similar to a ricochet of a bullet.... PING!. Usually found on pants that are WAY too tight, or well endowed women (usually followed or proceeded by Eee-Rrr if it is on a tight shirt.
Worker #1: Wow, check it out!!! Eee-Rrr, Eee-Rrr. PING!.
Worker #2: Tell me about it. Better get the safety glasses on in case that top botton comes loose.
Worker #1: Fuck that, get the body armor out!!!!