Flashcards will not help you in verbal. There is no set knowledge that is needed or helpful, unless you count knowledge of various verbal techniques. Verbal is focused on analyzing the passage in front of you quickly and accurately. You can't use flashcards to help you in either area. For instance, let's say you make a flashcard with a practice passage and try to glean the main idea as fast as possible. The problem is, it won't even help you the first time you use the flashcard because you would have already made the flashcard. In the process of creating the flashcard, you've probably already analyzed the answer enough for you to remember. At best, you'd only be familiar with the passage which is already a deal breaker.
Another way to think about the use of flashcards in verbal is what would happen if you retook the same verbal test again and again. Sure, you'd get all the answers right in record time and with little effort. Unfortunately, that would not translate to passages you haven't seen before. Retaking a verbal test enables you to memorize the test, not hone your analytical abilities.
The most effective way to practice verbal is to take TIMED practice passages. This way, you'll be used to seeing new passages, grasping the main ideas of each passage, answering the questions quickly, and dealing with the time constraints of the section.