Crown chakra is the one connects a person to the cosmic energy, the Brahman. It can be activated only by leaving the earthly attachments, after being drawn into a very deep state of meditation. Zen master Shunryu Suzuki said that Gautama Buddha was among the few people who activated their crown chakras.
Crown chakra is rather insignificant “during” the Yoga practice, because attaining it “is” the fulfillment of Yogic practice. The best explanation I know for the awakening of this chakra lies in Buddha’s description of Nirvana, and Paranirvana - that is the state of forever Boddhisatva established with the death of body.