"When this flask is uncorked by hand or shattered by being thrown, the fog that seeps out will temporarily cause the ground in the vicinity to vanish."
These potions are the reason that you should test unknown potions near the down stairway. If you accidentally drink this you will be able to walk right back up. They have the same effect as a pit bloat's attack, opening a temporary hole in the floor. The hole will close over several turns, filling in with empty ground. This means that Potions of Descent can be used to remove terrain such as foliage, water or even lava (but throw the potion onto the ground next to the lake, or it will burn up.
This type of potion becomes an excellent offensive weapon when ascending from floor 26 with the Amulet of Yendor. However, if you are still heading down, and drop a monster through the ground you will still have to face it on the next floor. Potions of descent will however permanantly destroy Goblin Totems, and Ogre Totems. Another way to fall through the floor is to stumble into a pit trap.