Murray designed a better door chain. The Securichain is fitted inside the door and the jamb, so that when it is closed the only part that is visible is a little box unit that sits on the edge of the jamb with a switch allowing you to easily engage or disengage the chain. The chain is like a little bicycle chain, making it very strong.
Murray and his team set themselves the task of making a door-chain that could withstand twelve shoulder charges by a fourteen stone man (or presumably one shoulder charge from twelve fourteen stone men - thus preventing most, but not all, the All Blacks from breaking into your house). They enlisted the help of the DSIR to test the unit and kept refining the prototype until it was strong enough.