The tendency to roll with application of rudder is not due to any momentary speed difference in the wings. Instead, when rudder is applied, the tail swings to point the nose in the direction in which the rudder is applied. This swing causes the airplane to move sideways through the air (yaw). If the wings have dihedral, the wing in the direction of motion hits the air at a greater angle (angle of attack) which increases its lift while the other wing has reduced angle of attack (less lift) so the airplane rolls opposite the direction of yaw.

This tendency to roll opposite a sideslip is called “dihedral effect”. While actual dihedral of the wings is by far the most powerful influence, other things effect this tendency. Placement of the wing on top of the fuselage will result in some dihedral effect even though the wing is flat. Placement of the wing on the bottom of the fuselage will have the opposite effect. This is the reason many low wing aircraft have more physical dihedral in their wings than similar high wing planes.

Sweepback will produce dihedral effect. This is why airplanes with a lot of sweep are usually built with negative dihedral. A tall fin and rudder will produce some dihedral effect,

In knife edge flight the airplane is in a continuous sideslip. The tendency to roll opposite a sideslip (dihedral effect) will cause it to try to roll back level and aileron must be held to oppose the tendency. In this case it is sometimes called “roll coupling”.

Some dihedral effect is desirable in trainers and general sport aircraft because the tendency to maintain level flight makes the aircraft easier to fly. In aerobatic aircraft which involve knife edge flight and other sideslip maneuvers it is generally not.