Typically, two one inch incisions are made at the corners of the victim's mouth. Then the victim is tortured causing them to scream. This tears the flesh along the sides of the mouth, leaving a "grin".
Can alternatively be called a Glasgow Smile .
Ichi the Killer has a Chelsea Grin.
Darren: Yeah alright, we can use the knife my mother got me for my birthday.
Cunt: Please, no, not the Chelsea Grin, anything but the notorious Chelsea Grin!