Top rudder refers to the rudder input necessary to keep the nose of the airplane from dropping while it's on it's side, while either rolling to inverted or holding knife edge flight. For instance, if I'm rolling to the right to inverted flight, I'll want to gradually deploy left rudder on the way around to keep the nose from dropping. As the plane rotates and approaches inverted, the rudder input is neutralized and some down elevator is deployed to keep the nose from dropping while inverted. The same scenario is repeated as the airplane is rolled back to upright level flight.