For non-NetHack players, this is a joke that results in a useful strategy. The potion of gain level normally causes you to get an experience level. But if the potion is cursed, the universe instead misinterprets the magical effect and you gain a dungeon level instead of an experience level—that is, "You rise up, through the ceiling".
This is generally very disappointing because an opportunity for your character to become more powerful was effectively squandered due to the curse. But magically moving upward quickly through the dungeon can be very useful if you need to escape from a dangerous monster quickly—or at the end of the game when you need to escape from the dungeon as a whole. The latter is what this speedrun used in order to avoid having to walk through the dungeon and take the stairs.
Most normal characters wouldn't be close to having enough potions of this kind to get entirely out of the dungeon by this means (also because of a certain complicating factor that prevents characters who can't control the RNG from knowing in advance exactly how many they'll need!). Although it's been done before by different forms of grinding, this is a whole other matter because the potions were obtained almost instantly with a preposterously lucky series of wishes.
This is generally very disappointing because an opportunity for your character to become more powerful was effectively squandered due to the curse. But magically moving upward quickly through the dungeon can be very useful if you need to escape from a dangerous monster quickly—or at the end of the game when you need to escape from the dungeon as a whole. The latter is what this speedrun used in order to avoid having to walk through the dungeon and take the stairs.
Most normal characters wouldn't be close to having enough potions of this kind to get entirely out of the dungeon by this means (also because of a certain complicating factor that prevents characters who can't control the RNG from knowing in advance exactly how many they'll need!). Although it's been done before by different forms of grinding, this is a whole other matter because the potions were obtained almost instantly with a preposterously lucky series of wishes.