It plugs in between your amp's speaker output jack and the amp's speaker. There's actually a 50 watt speaker inside the box, but no cone...so it makes no sound at all. It soaks up alot of the amp's output so the sound that comes out of the amp's speaker is lowered to any degree you set the attenuation to. I can crank up my 50 watt Marshall to volume 7 or 8, if I want to, and turn it back down again with Weber Mass. So I get the tone of a cranked Marshall at non-ear damaging volume levels. It's very cool and much more natural and lively sounding than a master volume control.