In this joke structure the key word of a sentence is reversed. Again, known phrases/cliches/phrases set up earlier in a comedy piece set an expectation to break.
Example, "He hasn't a single redeeming vice." The line, from Wilde again (he was a witty fellow) takes the standard phrasing "redeeming virtue" and the key word "virtue" is reversed to "vice." The line works even better because the punch of the joke is the last word.
In general, you want the line that gets the laugh to be at the end of a sentence. This gives the audience or reader a chance to laugh and not stumble over the joke.
Key reverses can be done for prefixes as well. The phrase "I was underwhelmed" began as a joke, though the word has now made it into our lexicon diminishing it's laugh potential.