The trance of possession is known as the "crise de lwa". Vodouists believe that during this process, the lwa enters the head of the chwal and displaces their gwo bon anj. This displacement is believed to cause the chwal to tremble and convulse; Maya Deren described a look of "anguish, ordeal and blind terror" on the faces of those as they became possessed. Because their consciousness has been removed from their head during the possession, Vodouists believe that the chwal will have no memory of what occurs during the incident. The length of the possession varies, often lasting a few hours but sometimes several days. It may end with the chwal collapsing in a semi-conscious state; they are typically left physically exhausted. Some individuals attending the dance will put a certain item, often wax, in their hair or headgear to prevent possession.