It allows the player to see one enemy's item drop, regardless of whether the enemy dies or not. The drop will be shown in an unobtainable form on the ground, similar to Boss Rush Mode. If there is no valid chest on the current floor, the item will not appear.
The first five notes give you a small backstory on each of these antagonists, but do not have any actual gameplay effect.
The last note, the "infuriating note", is different, as it takes the form of one of five overpowered guns that will be dropped by a boss if it has been defeated using the overpowered gun every time prior to its defeat.
The Infuriating Note is a reference to Trendy Entertainment's previous game, Dungeon Defenders II , where there was a ridiculously difficult achievement that required the player to find all six pieces of a map which then led to an optional boss fight.
The achievement was originally intended to take up to half an hour for completion, but ended up taking several hours as players traversed the worlds searching for the six pieces of the map.
There are six variants, each having one of these effects: reveal a way out of a maze, reveal a shortcut between two doors, teleport the player to another room, teleport an enemy or object to another room, summon an ally or object in another room, and poison enemies.
There are six variants of this item, each one revealing one of the six necessary directions through the maze to reach the Resourceful Rat's Lair.
The direction revealed depends on which room it is used in. For example, if Light Bullets are used in The Chest Chamber, they will show the way to get to The Chest through that room.
If an unused direction has not been revealed through that room yet, then it will reveal it there.