On the test day, after you finish your exam, you will be asked whether you want to void your exam or not. If you void your exam, it is basically the same thing as never having taken that test in the first place. Schools will not know you voided an exam. You will not be given a score nor receive a refund.
Most test takers do not feel confident about their scores after they finish their exams. If you have been scoring well on your practice exams, you should avoid the temptation to void your exam. It is normal to feel like you did not do well. However, if you know you did horribly (usually because you froze during the test or did not answer multiple questions), it is a good idea to void.