>>2021430Picture explains how it works for a building, but it works the same on the suspension by replacing the pendulum with a spring. Basically the suspended mass vibrates at the same speed as the mass it's attached to, but with enough delay to be out of phase. The vibration energy of the large mass is being damped by the relative displacement between it and the suspended mass.
https://www.youtube.com/watch?v=lhNjfNUOUo8The reason it's being considered alongside regular dampers is because there's too much unsprung mass relative to the total mass of the bicycle for a bike suspension to be able to absorb road irregularities without bouncing all over the place. It also has the plus side of being tunable for the specific frequency at which the tires bounce, while a regular spring + damper works more like a low pass filter.
The human body can work as a mass damper if the rider contracts and extends the legs at just the right time to counteract the oscillation, but that's impossible for frequencies higher than 1 hz or so, far below the response of the tires.
