Waterrockets describes this "weight the back wheel" thing in some long past post.

Some rear wheel movement is inevitable, but a skilled rider will be able to stay upright on extremely slick surfaces. Check out Petacchi sprinting in the rain - his rear wheel is moving around and guys around him crash when their front wheel just washes out. It's a recently past Giro, 2005 or something, where Petacchi was unstoppable. One stage ended in the rain and about 20-30 guys crashed including Bettini in the pink jersey. Petacchi not only didn't crash, he was sprinting really hard, really fast.

/\ what WR said