A reversal is a sentence writing technique in which the writer presents an idea in one form and then restates the idea in an opposing form.
Ask not what your country can do for you, ask what you can do for your country.
This sentence, from John F. Kennedy's 1961 inaugural address, does two things extraordinarily well: it uses parallelism and it violates the audience's expectations. The reversal sets up a direction, and then immediately disrupts it with the opposing phrase. The result is extremely memorable prose that sticks in readers' (or in this case, listeners') minds.
The Gettysburg Address contains an unusual two-sentence example of a reversal:
But, in a larger sense, we can not dedicate, we can not consecrate, we can not hallow this ground. The brave men, living and dead, who struggled here, have consecrated it, far above our poor power to add or detract.
And there's yet another excellent reversal in the Gettysburg Address--an even more powerful and forceful example of the technique:
The world will little note, nor long remember what we say here, but it can never forget what they did here.
The first part of the sentence sets up an expectation in the reader, the last part of the sentence abruptly violates that expectation. The parallelism adds cadence and makes the entire sentence memorable and distinctive. This is the kind of sentence an audience never forgets.