I've always called the clip smash an "insta-smash" because it happens instantly, without the full arm movement, which gives the opponent less time to react. Also, it can be used a lot more often than full smashes which usually require a high lift to give enough time to set up the smash.
I agree that Lin Dan's insta-smash is one of his greatest weapons, when right-handed players try to defend by doing a cross-court clear to his backhand side he just backpedals diagonally and intercepts it down the sideline; no other player can kill clears like that - it ends the rally right then and there, while in a match between lesser players such a defensive shot would cause the rally to last one more minute which annoys me a lot knowing that it can be killed.
For the ones where he backpedals I think it's because his extensive match experience and knowledge of player tendencies allows him to predict what shot his opponent will take and start moving a split second after they hit it, allowing him to get behind the shuttle. With lesser players, I notice that they would pause for a lot longer than a split second after their opponent hits the shuttle, during which it is too late to get behind the shuttle effectively and a result any insta-smash would lack power and leave them out of position.
For the ones where he doesn't have to move much before doing the insta-smash, I would say it is nothing special, a lot of players can do that and it usually doesn't end the rally right away. However I notice that many times Lin Dan would stretch and move his opponent so well that they end up lifting it right to him, without enough height and leaving a huge gap on their side of the court, which he then easily kills.