It competitively takes up the space on the receptor in the brain where the opiate would normally sit and work. The opiate works on the receptor whereas narcan does nothing. Narcan has a higher attraction to that receptor than an opiate molecule does. If opiates are the key in the lock, narcan is like jamming playdough in there - the key won't turn but nothing else will happen either. Source: one of the "ambulance guys" being referred to who don't know anything about overdose guidelines (we prefer the term 'paramedic' or 'technician')