It's a simplification of a bunch of ways to avoid route running techniques. Basically, coaches just don't want you to key on the upper body or feet as a simple head-fake or shuffle could cause you to over-commit.
I don't think they expect full focus on the receivers hips at all times such as when the ball's in the air. At some point you're going to have look at the receivers eyes and spot the ball yourself too. Rather, the point of keying on the hips is to not get fooled by a receiver's break. If he's running a post, your coach wants you to take the turn and drive of the hips as a sign that the receiver's made his break, not the shoulder and head fake he just made for an out. Once the receiver has made his break to commit to his route, the emphasis on watching the hips diminishes. He's going to look for the ball and you're going to now recover and close down the angle. Barring a broken play, the receiver is probably not going to work you one on one again as he's instead trying to provide a throwing target.
BTW, this is why receivers who can run a double move are often successful. They make one cut, changing their hip direction to sell the DB on one route and then drive hard into their real route. If they're quick and strong enough to make a successful second break, you will often be out of position because you did read his hips correctly!