You can do the front or rear elbow. It is the same principle as the jab or straight.
Steve Morris makes an interesting point in this regard – that if you are doing a rear hand strike it is offensive on the front foot, and vice versa for the front front hand. This is an incredibly powerful principle.
You can be offensive as well as defensive, in that you can shift your body forward whilst being on the back foot. Think about this carefully.
As far as practice goes, nothing beats relating to important concepts tirelessly.
