Hey VFR I'm not sure what side of bed Thomas Coupling woke up on cos he does not like Chadwick but his answer is wrong.
Chadwick is the name of the system designed by Mr Chadwick (of all people) from California to balance any rotating system. It is a digital analays system and is used throughout the world on blades turbines shafts etc etc.
On a helicopter the time comes in setting up sensors on the frame, main blades, TR blades, tail boom etc. Then you simply fly the helicopter in the regimes of normal operation, the computer does the rest and tell you where to place balance weights.
You land put the weights on where needed and voila.....a smooth chopper.
Other sysems such as strobe or bamboo and paint also work well!!!!!!!!