They allow you to choose two or three different rpm limits that can be activated at different times. With this ability, the possibilities are endless. As an example, we will use a drag car with a Three Step Module Selector plugged into the rpm socket of a 7AL-2 In. The different rpm modules are activated when 12 volts are applied to a corresponding wire. By connecting one wire to the line-lock circuit, one module will be activated during the burnout. This helps keep tire temps consistent. When the line-lock button is released, the limit turns off. When you are on the starting line, you can activate the second limit through the clutch or trans switch. When no modules are selected, the remaining high limit is active to protect the engine in the event of driveline failure. The Two Step works the same, but only with two different limits.