Well, when you say you're going to cut by 6db at 400hz, you *physically* push a button, turn a knob, slide a fader, click a mouse, or tap the keyboard to specifically tell the EQ unit, whether it be hardware or software, to cut by 6db at 400hz.
Take a more simple example. Let's say that you've got a volume knob labeled in decibels. Now, somebody says "turn up the volume by 6db", you just turn the knob by 6db. Nothing magic about it.
Now, how do people *know* when to "roll out 3db at 400hz?" Well, they just do. They know what they want to hear, and they have the knowledge and experience to know that performing a specific function will put them closer to where they want to be.
Visual aides can be helpful, like a frequency spectrum meter, but I don't think that most professionals would rely on a visual meter in most cases. They can be pretty helpful though, especially for learning. The best meters I've used on a PC are Pinquin Audio Meters.
Slackmaster 2000