They’re basically a literary prank– the sentence starts out in such a way that you think you know where it’s going, but the way it ends completely changes the meaning while still being a complete and logical sentence. Usually it deals with double meanings, or with words that can be multiple parts of speech, like nouns and verbs or nouns and adjectives.
So we get gems like
The old man the boat. (The old people are manning the boat)
The complex houses married and single soldiers and their families. (The apartment complex is home to both married and single soldiers, plus their families)
The prime number few. (People who are excellent are few in number.)
The cotton clothing is usually made of grows in Mississipi. (The cotton that clothing is made of)
The man who hunts ducks out on weekends. (As in he ducks out of his responsibilities)
We painted the wall with cracks. (The cracked wall is the one that was pained.)
