A magnetic motion detection method for a helmet system includes an internal body inside a helmet shell that emits magnetic flux and rotates independently. When the helmet shell undergoes rotational acceleration from impact, a magnetic field sensor on the internal body detects acceleration data of the magnetic flux. A controller processes this data to store it, transmit it, or generate triggering signals for airbag deployment when thresholds are exceeded. The airbag system rapidly inflates around the user's neck and subsequently exhausts gas based on predetermined delay or pressure increases.

A head protection device includes an airbag receiving case flexibly bent along a user's neck to allow both ends to be releasably fastened, an airbag folded and stored in the airbag receiving case, and an inflator part which includes a battery and supplies inflated gas to the airbag. The airbag includes an occipital part which protects a back part of the user's head, a front/upper head part which protects a front part and an upper part of the head, a right part which protects a right area of the head, and a left part which e protects a left area of the head.

A protection device including, a helmet configured to be mountable on a head of a wearer, an airbag housed in a folded state in the helmet prior to operation of the protection device; and an expansion source supplier configured to supply an expansion source to the airbag during operation of the protection device, in which the airbag is configured to cover from a posterior neck to both side neck portions of the wearer by expanding and inflating with the expansion source supplied from the expansion source supplier, and the protection device further includes an urging structure formed integrally with the airbag, and configured to urge the airbag toward at least one of the posterior neck or the side neck portions of the wearer by utilizing an expansion pressure of the airbag.