Understanding this verse depends first on our understanding Matthew 21:42 where Jesus describes himself as the "head of the corner," the capstone (not the cornerstone or keystone), which most translations also miss, but the real key is two words taken from the Greek Old Testament. the second verb creating a humorous image.

The play on words here uses the various meanings of "fall" and "on," and "before." The stone here is the "head of the corner," which has two meanings, the capstone, which tops and protects the wall from rain, and the head of community leaders (see Matthew 21:42 )." The point of the capstone is that it is slanted, a capstone in a corner being slanted in two directions, creating a sharp point. So the sense is the first fall, in the form of "the one falling" is that someone falling on the sharp edge of the stone, which Jesus in likening to his own sharp edges.

The verb translated as "broken" and "broken to pieces" is more grizzly. This word first appears in ancient Greek in the Septuagint, the Greek Old Testament, where it is used four times, mostly translated as "break" often in the context of smashing someone's skull, in one case with a hammer, so the sense is "altogether crushed."  Jesus only uses this word twice, in only appears in one other Greek work besides the Septuagint before Jesus.

The word translated as "grind to powder" and "crushed" is from a verb that means "to winnow" that is, separate chaff from grain by throwing up in the air.  Jesus only uses this word twice. It was not a common Greek word, but like the word above comes from the Greek Old Testament where it is used sixteen times, always used in the sense of "scatter" and "disperse" often against Israel's enemies. It does not mean "grind" in any sense.  This is the punchline, creating an image of someone exploding into dust, like a balloon popping.