Equally impossible to envision is how adding weight to the rear would help traction in a fwd auto. I simply edited the OP's statement as I felt the idea to add weight to the rear of a fwd car in order to increase traction was a typo.
I read the response that adding weight to the rear of a fwd car will be benifical as it will decrease the likelyhood of the rear 'comming around" I must say that if you are driving on a surface at speeds where the rear wants to come around you are simply driving too fast.
One way we made our off-road cars move through low traction situations is to keep the car as light as possible, I have seen the " keep it light" idea work very well. The heavier you are the more traction you need, I keep it light.
I remember once when the fire dept responded to a fire and the ground under this 20 ton fire truck got wet and the water on it froze. This fire truck did not go any where when it was time to leave but it certainly had enough weight if that was going to help, it did not help it at all.