Well, I missed the other thread. These things are called mass dampers or "vibration absorbers" and are used in all kinds of applications. Japanese cars are full of them.

If properly designed, the effect on the handlebars would be to reduce the amplitude of a vibration at a specific frequency. In an ideal situation the amplitude of a vibration can be reduced to zero. These things have to be properly tuned to the specific application otherwise they would be ineffective or even make the vibration worse.