How do you explain the car crash scene in Fight Club?

Most of the time, Jack (the Narrator played by Edward Norton) is in control of his body—sometimes without Tyler and other times with Tyler at his side. In these instances, any action that Tyler performs is actually taken by Jack. For example, the Narrator actually pours the lye onto his own hand.

But in the car crash scene, even though Jack's body was driving the car, Tyler was in control of it while Jack imagined himself sitting in the passenger seat. This is explained when Jack finally learns the truth in the motel room.

Tyler: Sometimes you're still you. Other times you imagine yourself watching me. (movie)

Normally, Tyler takes control of Jack's body when Jack falls asleep.

Tyler: But, when you fall asleep, I do things without you. I go places without you. Get things done. (script)

But Tyler also takes control at times when Jack is awake. The movie shows two brief flashbacks where Jack is acting as Tyler: when he threatens Commissioner Jacobs in the banquet bathroom and when he lays out the rules to Fight Club in the basement of Lou's bar. In these instances, Jack is still conscious and imagines that he's watching Tyler from afar.

Finally, for proof you need look no further than the crash itself. After the car comes to a rest upside down, Tyler climbs out of the passenger side window and pulls Jack from the driver side window.