Effects loop attenuator

[ZoomZilla]

Ok, I saw this thing that someone was selling which was basically a volume knob that goes in the effects loop. No, I don't want one, but my question is how it works. I use the send signal knob on my DR as basically the same thing, but what you are doing is lowering the signal from the preamp. So does this drive the power tube harder when you turn it up or not? There is some debate on this subject and I figured you geniuses could help me out. And I am not being sarcastic, most of you are very intelligent about amps.