I think the thumper control unit is a simplified version of the reflex unit in a Bally bingo. It appears to control the lighting of the pop bumpers, at least that's how it works on my Monte Carlo. As you win replays, it lights less of the pop bumpers as they alternate. MC has five pops, but they will only light in groups of two or three when the unit is stepped to the 'hard' position. The more coins that are put through the chute, the more it allows the pops to light. On the easiest position, it allows all five to light together, making the game easier to score on. I think it also controls how often the pops light. I hadn't realised it steps at a different rate up to down, but that's exactly how a reflex unit works. It take about 1 1/2 as many coins to return it to easy as it does replays to make it hard. You could even buy different ratchet wheeels to adjust the ratio. I reckon Bally just couldn't get out of the habit of controlling how easy or hard a game played, depending how it had recently performed. I'll guarantee some of the operators set the unit on hard and cut a coil wire or put some masking tape over the appropriate rivets, just to give the player less chance of winning. They did that, and much more, to reduce the chances of winning on a bingo!