The point, Brunvand notes, is not to do any real investigation but to simply have fun, and whether anything spooky happens - from a ghost appearing to a bird fluttering by in the night sky - doesn't matter. The fun is in the pretending: "Even if nothing happens, the stories associated with legend-trip sites continue to grow and develop as they are passed in the oral tradition of several generations of teens."
The legend of "Bloody Mary" is a common example, in which people (typically teenage girls) are dared to enter a dark room, either with or without a lit candle, stand in front of a mirror, and call Mary's name a specified number of times to summon a dead woman's spirit.
Depending on which version of the story is being told, Mary's ghost may suddenly leap out and attack the person who looks in the mirror. It's an obvious myth, but that does nothing to deter generations of girls from participating in the experience. In fact it's one of the most popular urban legends in the world.
Ironically most ghost hunters aren't familiar with folklore and don't realize they're engaging in legend tripping. To them it has all the trappings of a "real" ghost investigation or spirit hunt, and they claim to take it seriously.