An employee was getting a well head to stop on the down side and the weights on top. He was trying to roll up loose cable running from the rod rotator to a Samson post. His hand was on the brake and he was watching the weights when he pulled back on the brake to stop the weights. His hand then struck the top of the cross bar of the weight guards and his finger was struck in the middle. His right thumb was pinched and injured, resulting in a thumb tip amputation.