How does rudder lock occur? Citing Wikipedia article on the Vertical Stabiliser:


  Rudder lock occurs when the force on a deflected rudder (in a steady
  sideslip) suddenly reverses as the vertical stabilizer stalls. This
  may leave the rudder stuck at full deflection with the pilot unable to
  recenter it.


But how does this phenomenon occur from an aerodynamics point of view? What does it mean that the force reverses on the rudder? Is pedal force meant here? Furthermore, how can the pilot recover from rudder lock?
Normally, the airflow over the vertical stabilizer is from front to back, which means it hits the front face of the rudder, and acts to centre it. But the vertical stabilizer is actually like a small wing. In a sideslip, the airflow comes more from the side. If you're slipping tail-right, using left rudder, the airflow comes from the right. When the vertical stabilizer stalls, the airflow is no longer acting to centre the rudder: it's just pushing against it, and pushing it away from the centre.