Hit: "Charisma modifier thunder damage, and you push the target 2 squares. As a free action, an ally of your choice can make a melee basic attack against the target before, after, or during this forced movement."[D383:54]
Staggering note is an at-will power available to bards at 1st level.
Staggering note was introduced in the "Class Acts: The Bard" article by Arthur Wright in Dragon issue 383.