Most big backs have to get going North-South before they pick up momentum, Fui, Manase, Fahu, etc are all examples of this. But Harvey could maintain his speed on cutbacks because of his hips, which really helped with his receiving because it allowed him to make people miss in the open field.

I think Jamaal has much more upside than Harvey due to his game breaking ability, but Jamaal has been nowhere near as dynamic in the passing game (other than a few shovel passes) and Harvey was much better in pass protection as well.