Like most standardized tests today, the NCLEX is administered by a computer. The NCLEX uses computer adaptive test CAT which reacts to the answers you give to determine your level of competence. The selection of questions is based on the NCLEX-RN test plan and by the level of item difficulty.
Every time you answer a question, the computer reevaluates your ability based on all the previous answers and difficulty of those test items. Your first question is relatively easy, if you selected a correct answer, the computer supplies you with a more difficult question from its question bank. If you have selected an incorrect answer, the computer gives you an easier question. This process continues throughout the examination until the test plan requirements are met and the computer is able to determine your level of competence.
Additionally, there is no option to skip a question, you must answer it or the test will not move on. You cannot go back and review previous questions and change answers.