Hexes were an extension of the machine nuts that climbing started using for protection. They eventually became ‘hex centric’ in that each side was of a different length. The reason behind this was that someone trying to make their own at home wasn’t very accurate as cutting and made it by accident as realised that the eccentric shape work even better.
Hexes are a semi-passive piece of equipment, as although they are just placed into a crack. When they are put under load however they torque into the crack. Also actively forcing themselves into a better placement. Often in order to retrieve them from a placement the climber will have to turn them in the opposite direction and you will hear and feel an audible click and the hex will become loose.