How are joysticks 'powered' in gliders?

Don Johnstone wrote:
...
In most modern gliders the stick forces are very light,
frightening so in something like a Discus. Some of
the older gliders did get heavy at speed, for instance
at 80kts the ailerons on a Sedbergh (Slingsby T21)
are set in concrete requiring two hands or even four
if available, not to mention the foot behind the stick
to relieve the need for forward pressure on the stick.


What is it that causes the difference between the two?

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FF