King K. Rool (SSBU)/Down smash
A belly flop, emphasising K. Rool's huge weight and armor. King K. Rool's down smash is among his most notorious attacks, for its Belly Super Armor and hurtbox shift specifically making grabs miss. This is because K. Rool's hurtbox shifts above grab hitboxes as soon as frame 4. While it deals less knockback than forward smash, it remains a monumentally powerful attack, ending stocks as early as 80%. Due to this, K. Rool's down smash is largely considered to be one of the best down smashes in the game.
Down smash is one of the most difficult moves to challenge on startup in the game, due to the aforementioned hurtbox shift and Belly Super Armor. Low attacks and regular grabs simply miss, and regular or high reaching attacks tend to hit the armor. The only feasible way to challenge the move is to attack from above, where K. Rool completely lacks any armor. This protection, combined with kill power, makes down smash - long and far - one of the best hard punish tools in the game. It's not uncommon for K. Rool players to watch for greedy dash grabs and use the information to punish poor approaches later, simply because the reward for doing so will be substantially higher compared to other characters, all thanks to down smash's power.
It's also notable to mention that down smash deals significant shield damage, so much so that use after a neutral aerial on shield is a guaranteed shield break. This notably avoids grabs due to the safety of neutral aerial as well, making a perfect shield or a roll backwards the only way to avoid it, which in high-pressure situations may not be thought about. However, with repeated use, it becomes better to prepare for these options instead, due to the punishable nature of down smash against an experienced player.
Unlike many of K. Rool's attacks being designed to be difficult to challenge but easy to punish, down smash is reasonably well-defended even after the initial slam is over. It has a small quake hitbox just outside of the body slam's range, much like down tilt's. This quake gives sizable frame advantage on hit, but generally not enough for solid followups outside of forward tilt. However, if shielded, this quake hitbox doesn't have much shieldstun at all, leaving K. Rool dangerously open regardless; the quake is -34 on shield, enough for even charged smash attacks to be valid punishes.
Down smash typically isn't used outside of reads and hard punishes, however. This is because the move is quite slow, with Belly Super Armor not switching on until frame 11, and the hitbox coming out frame 22. It has an enormous FAF of 64, though this is the middle ground in terms of K. Rool's smash attack FAFs. On top of this, in situations where down smash can be done for free early stocks (such as shield breaks or highly punishable moves are used), forward smash angled upwards would KO much earlier, making it strictly suboptimal. Furthermore, if Belly Super Armor is damaged, K. Rool becomes very vulnerable: it has 14 active armor frames in total, which is enough for at least some heavy hitting attacks to hit it, especially considering how the move is used. As a result, many armor breaks occur from down smash misuse. As a result, in higher level play, down smash tends to be used far less on average.
Contrary to popular belief, while K. Rool leaves the ground as soon as frame 4, he maintains a foot hurtbox extension until frame 7, which makes it deceptively easy to grab him from behind. It is even possible - in fact easier - to grab K. Rool out of down smash when he has actually began the belly flop. This is commonly seen with tether grabs such as Samus', or large pivot grabs such as Greninja's. Command grabs such as, humorously, K. Rool's Blunderbuss Vacuum, can also force him out of it.