There has been report of sudden spontaneous opening of the crown of persons who were not even on any spiritual path. But I believe that is rare. In AYP the opening happens gradually, in a safe and (mostly) stable manner and is permanent. AYP, through the practice of Spinal Breathing Pranayama, goes for a universal purification of all chakras, not singling out any particular one. But what does "an open crown" truly mean? It is not about being "tele transported" into higher realm but rather perceiving and living as That, the state of Oneness or non-duality. It is described in many different ways.