Being able to effectively apply what you already know to a new topic that you may or may not be familiar with, based on the background information they give you in the passage. There's also being able to analyze the answer choices and determine which ones are not likely to be correct. Basically, its an extension of the concept of test-taking skills. If you don't have either the fundamental knowledge required to take the test nor the critical thinking capacity to use them in a limited amount of time, you aren't going to do that well.