In Harbrace's 12th and later editions, Chapter 22 is entitled "Clarity and Completeness -- Include all the words or phrases necessary to complete the meaning of the sentence."
In previous editions, at least through the 8th, the Chapter heading was "Omission of Necessary Words -- Do not omit a word or phrase necessary to the meaning of the sentence."
Isn't it uplifting to be positive in our grammarianism? In this case, I prefer the old heading: I think it expresses the problem and the solution better, even though it's -- gasp! -- negative.
Anyway, various dropped words have come up in SAT Questions of the Day, including --
-- the need for a "the" to complete a parallel construction [rule 22a(1)];
-- the need for different prepositions after two different verbs [22a(2)] -- "I neither believe in nor approve of those attitudes.";
-- the need for necessary auxiliary verbs [22b] -- "Dieting has never been and will never be a complete solution to obesity.";
-- and the need to complete comparisons [22c]-- "Most people think television is better than it used to be."
A visit to the five pages of Harbrace Chapter 22 will alert SAT-takers to several sneaky "omitted word" questions, which are otherwise very easy to miss.