To try and improve the front forks on my old GPz have read about these things - go inside, sit on top of the damping rod, with the spring on top and are meant to work as a valve to improve the suspension and make old-fashioned spring+damping rod forks behave more like modern units that come with cartridge forks.

Racetech is the only company making these things (from the USA) - has anyone used them and are they any good? Have put in a Hagon progressive spring that on smooth tarmac has improved the handling but not on bumpy roads.