Here are the very basics needed to get a good knife edge.

First is the design of the plane. Large fuselage side surface area, symetrical wings with little or no dihedral, large rudder that has longer chord at bottom, and good engine power and proper thrust angles are all found on good aerobatic designs and contribute to good KE. Compare a Cap to a trainer.

Center of gravity CG is next thing to understand. If your plane is nose heavy and you must use up elevator trim to maintain upright, level flight, when you roll to KE, that same up elevator trim makes the plane pull to the canopy, out of KE. Tail heavy is just the opposite. Improper engine thrust angles can do the same thing to a lesser degree.

Yaw/roll coupling is next. To see roll coupling, in upright and level flight give rudder only control input, if your plane starts to roll the wings into a bank, the rudder is causing it to roll. To see yaw coupling give aileron only control and watch the plane turn and change heading as the wings bank/roll. Only the best acrobabic planes are designed without this coupling. Computer radios use premixes to program a small percentage of the opposite control to eliminate the coupling. For example, if the plane rolls right with right rudder only input, mix in a little left aileron to keep the wings level when you use rudder.

This is why the aerobatic trimming charts are so important. They take you through the whole process step by step to properly trim the plane for aerobatics. The idea is to trim the plane so that you can find the "sweet spot" in KE where the plane is able to pretty much able to stay where you put it, especially in KE. It is almost imposible to KE when you have to constantly manually correct every control to compensate for poor trim set up. You’ll be surprised how easy knife edge is with the proper plane properly set up, and why it is almost impossible with the wrong plane or wrong set up.