Kubo pulled out all the stops in designing Kenpachi’s shikai, and the result is the biggest and most ludicrous-looking sword we’ve seen yet. A sword is perhaps the wrong word to describe it, as it’s more of a giant axe-cleaver hybrid, taller and wider than Kenpachi himself, with a tassel on top. Supposedly his zanpakutou has always been in permanent shikai form, but he has to call his sword’s name and command (“Drink”, Nozarashi) to release the “true” shikai form; this is a bit of a contradiction, especially when compared to Ichigo’s shikai. Alternatively, “drink” could refer to the specific ability he uses to destroy the meteorite, possibly by absorbing its power and transforming the sword.
It’s not completely clear how he stopped the meteorite as all we see are fiery remains raining down on Sereitei, but I’m pretty sure his shikai provides more utility aside from just boosting his brute power level. Interestingly, the other Sternritter are actually very concerned about the city-sized meteorite which penetrates the shakonmaku easily, but Kenpachi isn’t even fazed as he destroys effortlessly. Hell, it looks like he’s actually getting more and more excited, and Gremmy had better use both of his brains to come up with something a lot better — right now. If Kenpachi decides to take off his eyepatch, wield his blade with both hands, or release bankai, then this battle will be over soon. Gremmy’s time to live is numbered in seconds.