There are a couple of pictures in the gallery that demonstrate how it works.
Here's the back of the game in its "balanced" position. When the handle is
pivoted, the horizontal beam tilts in one direction or the other. The challenge for the stagehand is to pivot the handle back and forth quickly enough to be somewhat realistic and exciting, but not so quickly that the "pans" swing too far and smack into the vertical pillar in the center.
I don't remember whether I've mentioned this before: in the first picture, see that empty rectangle above the handle? The one that looks like it could fit a four-digit display? When the game was built, that's where the price readout was. Then they discovered that when the scale was unbalanced, the beam either partially blocked or was too close to the display, so the display was moved to one of the tables.