The 950B stripping coal.

Though it's a little hard to see in this picture, the dipper and moveable counterweight are both connected to the hoist drum. They are spooled in opposite directions, so that when the dipper is being raised the counterweight is being lowered, and vice versa. This allows the dipper to be raised with less "generated power", the momentum of the falling counterweight helps to pull the dipper into the material being loaded. This also allows for rocky materials to be loaded easier (The momentum of the falling counterweight provides the breakout force at the bucket's teeth rather than the hoist motors). This greatly reduces the amout of "work" that must be done by the hoist motors, thus reducing wear and extending their service life.