with a tap; with a knock (the sound or manner of light tapping).
誰かがドアをとんとんと叩いた。
Someone knocked on the door (with a light tapping).
テーブルをとんとんと指で叩いて合図した。
I tapped the table with my finger as a signal.
smoothly; without a hitch (used when things proceed effortlessly).
話はとんとん拍子に進み、契約が決まった。
Things progressed smoothly, and the contract was finalized.
準備が整って、とんとんと計画が進んだ。
With preparations complete, the plan moved forward without problems.
even; equal; break-even (indicates no advantage for either side).
ゲームの得点はとんとんで、最後までどちらが勝つかわからなかった。
The game's score was even, and no one knew who would win until the end.
売上と経費がとんとんで、今月は利益がほとんど出なかった。
Sales and expenses were about even, so there was hardly any profit this month.