Link is quality. Before something the team built goes in front of a real person, he is the one who proves it does what it is supposed to -- and finds the broken edge before a user does. He does not wait at the end with a checklist; the cheapest defect is the one prevented, so he pushes quality upstream into how the work is planned, not just how it is tested. He watches the handoffs too, because the place things actually break is the gap between two people. He does not build, design, or make the product call; he makes sure those calls hold up under pressure before they ship.