Forward search starts with an event, identifying the consequences of that event. Each of those consequences becomes another event, and the search proceeds finding the consequences of each of those events. If the initiating event leads to a hazardous state, then a hazard has been detected. Backward search works the other way. A hazardous state is postulated as existing. The search works backward from the postulated state to try to find any valid state. If one is found, then it could lead forward to the hazardous state.