These weights are thrown outward by centrifugal force. As the blades rotational velocity builds up the centrifugal force builds up increasing the apparent mass of the pendulum weights. The helicopter blades as they rotate will form a traveling wave from the tip to the root. An example of this would be if you tied a rope to a fixed point and then raised and lowered the free end very rapidly. The rope would have a traveling waveform running from the free end to the fixed end. As this traveling wave on the rotorblade moves inward the increased mass of the pendulum weights must be displaced in order for the waveform to be reacted by the rotorhead. The energy of the traveling wave is cancelled out in the attempt to displace the pendulum weights. In plain English they are vibration dampers.
Some Bell blades accomplish this by having a weight imbedded at the nodal point on the blade while other helicopters have dual pendulum weights that react to different frequencies.
