With the wings vertical in reference to the ground, there is no vertical lift except for the fuselage.(lift is always perpendicular to air flow over the wings). The shape of a fuse does not provide as much lift as a wing but if designed for knife edge flight, it will be enough to hold steady and even maneuverable enough to turn,climb and go up or down.Its also best to have a fully symetrical airfoil for knife edge flight,this type of airfoil can have 0 lift at zero angle of attack, which requires less control inputs to maintain headings.
With that in mind,look at some of the best knife edgers-pattern planes,H9 Showtime,Hyde Vision,Ultimate Bipe-all have the fuse and SFG's to make knife edge easier.(there is more side area up front to make the in-flight CG forward and more controlable).To take it to an extreme, the U-Can_Do has a tall fuse up front with the canopy way up there and a very narrow fuse too,it can hold knife edge at slower speeds than most other planes.
When first starting to fly in KE, get the plane up to safe height,roll 1/4 roll until top of plane is facing you and keep it in that attitude to observe what happens(full throttle at first). Most will start dropping some, to maintain a steady altitude, rudder must be used since its now horizontal like the stab usually is when upright.Once the right amount of rudder travel is achieved, the plane will hold level flight and then you can start adding other controls to turn or keep the plane from rolling if needed.Start out by just flying straight lines at first, then later you can try turning/climbing in KE or 4 point rolls.