tap down
To drive or pack something down by repeatedly striking it lightly and swiftly. A noun or pronoun can be used between "tap" and "down." Just tap the piece down gently. You could break the whole machine if you hit it too hard. He tapped down the tobacco into his pipe and lit it with a match.
tap something down
to pound something down with light blows. Please tap that nail down so no one gets hurt on it. Tap down the tack, if you would.