The mod is exactly what it sounds like, it adds a Fus Ro Dah! Press Z (default) to activate. It just pushes nearby mobs up and far away into the direction you are facing.
Once you’re done FUS RO DAH-ing cows off of cliffs, you may want to check your .minecraft/config directory for fusrodah.cfg. In there you can change the “power” of the FUS RO DAH and the up-lift. Lift controls how much vertical momentum mobs are given, power controls horizontal momentum. If you’re feeling like a troll, I would set the power to about 2 and the lift to around 10. Note: decreasing the power also decreases the “hit area” of your FUS-RO-DAH, so I wouldn’t recommend anything lower than 1.5. You can also enable or disable the sound being played every time or change the sound to be longer or the original sound that was added for this mod.
I don’t limit valid input so if you really want you can set your power to 100… although this will probably result in every mob even remotely close to you being blown out of your render distance. If that’s your goal, have at it.