It's not lowering the power pe se, it's limiting how much of the actual max board power you re allowed to draw. So if the board max power is 200 watts then by changing the slider to 80%, you are changing board max power to 160W.
This means you would get lower clocks with something like furmark, but with mining it's much easier on the GPU and doesn't really use every component at max full, so of the card will operate just fine at 100% at this task with the power limited.