A control surface on an aircraft, used primarily to induce yaw or to counteract adverse yaw
while applying aileron to roll; proper use of co-ordinated controls.
The yaw causes the wing on the outside of the turn to fly faster than the other, thus increasing
that wing's lift and rolling the aircraft into the turn. That's the secondary effect for rudder:
roll. If you wish to perform a flat turn, eg as a too fast signal while launching a glider with a
winch, you will need to use crossed controls, ie apply aileron opposite to the rudder.