The grip strength of curling your fingers into the base of your thumb is stronger than the pinch strength of pressing your thumb against your fingers. Scissor

The grip strength of curling your fingers into the base of your thumb is stronger than the pinch strength of pressing your thumb against your fingers. Scissor ergonomics aligns the handles so applying the finger-to-base-of-thumb grip to the handles (with the upper handle pressed against the outside of your thumb’s knuckle) pulls the blades together. In order to do that, scissors have to be designed to fit either a left or right hand. To use scissors wrong handed you have to catch the outside of your finger knuckles against the lower handle and pinch the upper handle down with your thumb.