Usually when you hold down the Command key it "gears down" things so you can make finer adjustments. For example say you're lengthening or shortening a clip in the timeline, if you hold down Command you can adjust it frame by frame (if you're using the mouse that is).
But, it's the exact opposite if you're in the Color Corrector 3-Way filter. Normally when you drag around the indicator dot in the color wheel in this filter it moves fairly slowly. But if you want to drag it around to make broad changes in color almost in real-time with your mouse, hold down Command and in this rare instance it "gears up" the movement.