This is a plugin for Details!, if you don't have it, install using the wago.io addon updater.
What this plugin does:
Tracks your current target and show in the window who is doing damage to it. The amount of damage shown is the amount dealt since you selected the target.
When this plugin can help me
In Raids and Dungeons: show how fast players are switching targets after you select & tell players to attack the new target. You also see in real time how much damage the target is receiving.
In Battlegrounds: show if the players are respecting your calls to attack a new players on a RBG.