What does the counter balance do?

 

Let me see if i can explain this right.It is pretty much there to counter balance the rotation of the crank shaft as it is turning.It actually rotates at the same rpm as the crank.It is also there to help with vibration.If it wasn't there it would probably vibrate your fingernails off.This is the way i understood it to be anyway.Not too good of an explanation but that's the best i can do.Hope this helps.