Glider rudders need a counterweight to be properly balanced.
What?! Aren't airplanes supposed to be light? Why add more weight?
This reduces the tendency toward flutter. (Any airplane, at a high enough speed, will flutter. Don't go that fast.)
An unbalanced rudder (or any flight control surface) will be prone to flutter at slower speeds.
1st photo shows the rudder on a bracket that allows the rudder to freely pivot. (See alt text for more discussion.)
2nd photo shows rudder oriented normally. (See alt text)
https://en.wikipedia.org/wiki/Aeroelasticity#Flutter
Imagine being the test pilot here...deliberately gaining speed to get (just barely) into the flutter regime...surviving, and learning.
https://www.youtube.com/watch?v=kQI3AWpTWhM
I am not using a control horn to reduce the control pressure I feel on the rudder pedals. Modern gliders typically don't need that.
https://en.wikipedia.org/wiki/Flight_control_surfaces#Control_horn