It's a cylinder with a spring that pushes a ball into a cone at one end. The water pressure opens the ball (where/when depends on what pressure your cooling system is pushing at a particular rpm) and lets water into the stinger of your exhaust. You can add washers to stiffen the spring and require more pressure to open it. With little to no water entering the stinger at idle, the engine doesn't need to push excess water out of the waterbox which gives better throttle response off idle. As your rpms increase, more water is let in to control temperatures. Art makes an adjustable (pro model) controller and non adjustable. The pro model doesn't require the washers, you just twist it tighter to add tension to the spring.