Remaining in the trance is essential for natural birth. You need to create a bulk around you and stay there. However, a birth bulk is fragile - many factors can weaken or break it.
It is not easy to remain in the trance while a midwife is handing out a form to fill, speaks to you during a contraction, let alone performs a vaginal examination. This all can be avoided if you prepare your birth plan and instruct your birth partner on how important it is to be undisturbed. Also, vaginal checks are not necessary in a healthy pregnancy and natural birth - read a bit about it and consider whether you really want to agree for them.
Do not plan on communicating too much - remember that speaking in this stage will require significant effort and you will need to spend your energy carefully. Avoid waking up your left brain - you need your instincts, not logical thinking right now.
Ironically, or rather sadly, countless doctors and midwives do not know about the trance. Some of them probably never saw a truly natural birth. You cannot count on their cooperation on them in this matter - this is why a part of your preparation should be spent on planning how to create a calm and undisturbed atmosphere.
References
Undisturbed birth: http://chriskresser.com/why-undisturbed-birth
Vaginal exams during birth: http://mamabirth.blogspot.com/2010/12/obstetric-lie-100-failure-to-progress.html
Dangers of vaginal exams: http://www.natural-pregnancy-mentor.com/vaginal-exams.html
