I don't know much about how it all works, but I thought it was along these lines:
The Carbs provide the energy to your muscles so you can lift more without your muscles failing, and therefore tear the fibres better, allowing for more regrowth and size gain.
The Protein is what repairs the muscle tears, making them slightly bigger.
Sugars just help the movement on these nutrients around your body.
If this is true, then why should protein be eaten before a workout, and why carbs after? It makes sense that carbs are required around 90mins before (based on the nutrition thread), in order to give your muscles energy for the workout, and protein after, to start repairing the damage you did IN the workout. But I can't see how Protein prior would be a benefit, because your muscles arn't torn yet and there's nothing to repair, and how carbs after would be, because you've already done the workout and are no longer requiring the energy to lift more.
Cheers for the help