The easy answer is the fuselage gives the lift. The involved answer includes power settings, vertical stab and rudder and the canopy. On a biplane the interplane struts, out near the end of the wings, helps allot and gives the rudder much more authority.

Take the H9 Showtime with the Side Force Generators (SFG) as an example. Fly it without them installed and you need a a few degrees of rudder to hold a "normal" nearly level knife edge. Install the SFG's and now you need very little rudder to do the same knife edge.