Quinch, or Quinching, is when a female is about to Quiff (fart out of her vagina) but pinches it, or stops it, from coming out by contracting her vaginal cavity with such intense force an actual diamond could be made by sticking a piece of coal into it mid quinch. Also usually met with an intense or ridiculous facial expression
Shawn: DIANE! Did you see Amy's face?! She def just quinched.
Diane: What the christ is quinch?
Shawn: You knowwwww...when you fart out of your vagina and the you stop it from coming out...?
Diane: Fuck you