A Ford Mustang skids around a corner, its screeching wheels harmonising with the roar of its engine. Hot on its tail, gripping the steering wheel of a yellow Dodge Challenger RT is John Tanner; the wheelman. Pressing his foot to the floor, Tanner urges the car forward, eventually bringing it alongside the Mustang. The two cars tear down the middle of a busy highway, weaving amongst oncoming traffic like mechanical snakes. Taking his eyes off the road for a mere second, Tanner tries to catch a glimpse of the driver in the Mustang. The blast of a horn brings his attention back to the road; Tanner's windscreen is filled with the sight of an eighteen wheeler truck. With no time to react, Tanner smashes into the vehicular behemoth at a terrifying speed. The chase was over.

Except it wasn't. Martin Edmondson, creative producer on Driver: San Francisco, and the man demoing the game to us had a pretty nifty trick up his sleeve. Pressing the X button, he shifted the view from that of Tanner's Challenger, to a top down view of the city. The action on screen continued in slow motion; I could see the truck still pushing the yellow carcass of the Dodge down the road. Just what was going on here? Was Tanner dead and having some kind of out of body experience? Moving a reticule around the screen, Edmondson hovered over a nearby Aston Martin, and suddenly the camera jumped into the car. He was back in the action again, behind the wheel of a new car, pursuing the same Mustang down the same stretch of highway.

I can imagine the blend of disbelief, confusion, and bewilderment on your face right now, because mine looked exactly the same as Edmondson explained the Shift mechanic to me. To put it simply, Tanner is able to shift his consciousness into the driver of another car. In effect, Tanner becomes a completely different person each time he shifts from one car to another. When in shift mode itself, Tanner floats around the city like some kind of omnipotent being, and can move around the city far quicker than any car. While this happens, the 'real' Tanner is still in the game world, but in a trance like state. It sounds ludicrous, but Reflections has been sure to support the crazy mechanic with a plausible narrative. In fact, the mechanic is integral to the very plot of the game.

After the events of Driv3r, John Tanner was left in hospital in a critical condition. This is the point at which the new game picks up; with Tanner lying in a hospital bed in a coma. The weird thing about Driver: San Francisco is that the whole game actually takes place in Tanner's mind. If, like me, you're worried that this will effectively make the whole plot redundant, have a read of my interview with Martin Edmondson, who explains exactly why the narrative is so significant. In his comatose state, Tanner is still an undercover cop, except he thinks that he's gained a strange super power of sorts. With a quick tap of the X button, players can pull Tanner's consciousness from his own body, and shift it into the body of any other driver in the game.

Players can't shift from car to car willy-nilly, however, the game takes advantage of a resource system that ensures players make use of the mechanic sparingly. By drifting, narrowly avoiding oncoming traffic, catching air and generally driving like a badass, Tanner (or the person he's possessing) will fill a resource meter. This can then be used to shift between other cars, with the meter gradually depleting based on how long you stay in Shift view.

With no running around outside of the car to break up the action, Driver: San Francisco puts the emphasis firmly back on driving. Good thing too, because Reflections has licensed some 120 cars for use in the game. Ford, Maclaren, Aston Martin, Alfa Romeo, Lancer, Pagani, Mazzarati, Audi, - the list is exhaustive. So too is the city of San Francisco itself, which features some 208 miles of driveable road. Edmondson was quick to point out that this makes Driver: San Francisco the largest open world driving game ever, and that it would take over two hours to traverse all of its roads at a constant speed of 100mph. Of course maintaining 100mph isn't easily done, so it would likely take much longer.

Certain vehicles you occupy come complete with their own missions. Shift into a police car for example, and you can engage in frantic pursuits of criminals mid getaway. At any point during this pursuit, you could choose to shift into the criminals' car, and escape from the same police cars you'd just been driving. While we only really got to see the police missions, it's safe to assume there'll be taxi, ambulance and fire engine missions too, amongst others. A serious amount of writing has gone into bringing these characters to life, and shifting into a new car will reward players with an insight into these people's lives.

The game looked great too - it didn't blow me away, but it certainly looked good. Edmondson promised that the game would run at 60fps when it was finished, but admitted that the version we were playing was only running at about 50. San Francisco is a fantastic choice of city to play host to the action. With its bustling highways, narrow alleyways and rolling hills; it's a driver's paradise. Edmondson was clearly in love with the city too, explaining that his decision was influenced by films such as Bullitt, which saw Steve McQueen in one of the most celebrated car chases of all time. Driver: San Francisco is all about these iconic car chases. It's not about mowing down pedestrians and killing prostitutes, it's about recreating the epic car chases inspired by Hollywood.

Most of my time with the game was actually spent in the multiplayer, with a mode called Trail Blazer. Here drivers are tasked with following the trail left by an AI car; with points amassing the longer they are able to keep up with it. In multiplayer the shift mechanic really comes into its own, with players simultaneously flitting from car to car, desperate to get hold of the hottest set of wheels on the roads. With the build we were playing, that car was the Pagani Zonda, and as soon as one appeared, everybody would enter Shift mode in a desperate attempt to snag the beast before anybody else. Those that failed to do so would be left floating around in Shift limbo, forced to find another car while the opposition is off scoring points.

Driver: San Francisco looks good, handles well and features one of the most innovative mechanics that has graced the driving genre for years. In principle it might sound ridiculous, and it probably needs to be experienced firsthand to appreciate properly, but it offers an incredibly refreshing take on open world driving. Only time will tell how well the writers wrap the narrative around this mechanic, but it's certainly going to get people talking.

Driver: San Francisco is due for release on Xbox 360, PS3, PC and Wii Holiday 2010