From the Super Mario Wiki
Drop Blocks are a type of block Wario encounters during the events of Wario World. They are mostly found in the Wooden Trapdoor and Steel Trapdoor sublevels, and look like floating, green blocks. Drop Blocks are always stationed in mid-air, and when Wario steps on one, the weight causes it to descend slightly, pause, and fall quickly. If Wario is on the Drop Block when it falls, he may be forced to start the sublevel over, particularly if he's inside a Steel Trapdoor's sublevel, which has no ground. The Drop Block will return to normal position if Wario steps off it.
Obviously, Drop Blocks, like Marshmallow Blocks, must be crossed quickly.