A douhini is the tickle of the inside of an exposed armpit. The Douhini-er must then yell Douhini to further surprise the victim. Usualy accompanied by a slight waver of the pointer finger. In other cases a Douhini can also be a jab to the armpit but that technique is usualy frowned upon.
As Isaac lifted his arm behind his head he heard the words, "Douhini." Then felt a slight tickle in the armpit.