practicing holding a plane steady by not loosing height in every position while a plane is rolling. whatever it takes to keep a plane off the ground while inverted i.e. a small amount of elevator depending on how you have your bird balanced. what position it takes for your plane to hold this/that side of knife edge i.e. left or right rudder. the hardest thing will be for you fingers to keep up with the correct inputs, the timing will come with practice. it takes more rudder than elevator again depending on how you have your planes balanced, I have most of my birds balance for hands off or very very close to hands off inverted so very little pushing or pulling is needed on the elevator. The more side areas you have on a bird the more forgiving it will be on the knife edge side of the roller.