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the front axle has a kind of freewheel in it, looking at the sprockets it looks to be a slightly reduced gear ratio, but hard to say.....so when the rear wheel has full traction it is pushing the front wheel (spinning faster then the gear drive to the front, freewheel), it's when the rear wheel spins that the front wheel is moving slower than the rear and the drive takes over and pulls the bike, it works pretty seamlessly on dirt i'm told and the bike just hunkers down and goes.
i believe it was yamaha who had a similar system a while back but it used hydralics to drive the front gear.....same principle though, as when the drive line would spin faster (when rear wheel spins) the hydraulic pump would spin faster and drive the front wheel more.
i think the chain/shaft drive system is more efficiant, i'll see if i can dig up info on the yamaha setup.