Each and every Tel-Ray we tested (more than a dozen!) varied drastically in motor and flywheel stability, resulting in different pitch and variation effects. The same unit even sounded different day to day, depending on temperature, warm-up time and other factors.Since the original units are basically thirty year-old tuna cans bolted to plywood with springs and motors flopping around inside, the Variation knob was added so you can dial in a Tel-Ray in whatever state of disrepair you desire.