It's not necessarily a significant event that happened, but the emotion behind it. Strong emotions felt during the day sometimes do manifest themselves during dreams. Unfortunately, it's negative emotions like fear that tend to have the most effect on us. I guess this method could work for some people once or twice if they're really desperate, but fear is not something to invoke repeatedly, purposefully, it's unhealthy from many points of view. It's not reliable either, a person might get scared the first time, and maybe the second, but it's unlikely to happen from then onwards.