If you set up a button, a piston and a redstone dust in this composition...
The piston will be activated while the button is on (and will retract when off).
However, I can't find the rationale why this gadget works. I guess this is some kind of BUD mechanism, but still don't understand what route the piston is powered through.
As far as I know, if I remove the dust below the button, it'll no longer trigger the piston.
Nor when the dust is powered otherly.