I have never pulled or jerked the leash before and I don't know how effective it is, but I see many people on the streets overusing it to the point of harm. I know the poster above me mentioned *light* pulls though.
Anyway, reading this thread reminded me of a vet article I recently read about collars and leash pulling/jerking:
Dog collars can cause disease and possibly lead to cancer « Dr. Peter Dobias – holistic veterinary medicine