When you roll consider that two forces are pushing or pulling you in different directions (or more than two) and relax and allow your body to find the logical solution while staying as relaxed and aligned as possible so you can continue to move and breath freely. For example if you are pushed back in your forehead and sideways from within on the thigh, the body goes with both directions while adding his direction and completing this with either a step or a roll. In the beginning it is good to use hands and fists and later on move to sticks and knives. Remember to also grab a part and apply pressure somewhere else or vice verca so the partner learns to avoid making movement which are redundant such as throwing himself.
On a personal note: if you do the slow movement and rolling combined with breathing you are pretty much covering all bases in solo work.