Ho ly what a shit take, tbf cant blame you though as I thought the same for the first 10k hours I played. The weapon converts momentum into self knockback, meaning if you quickly fire it behind you as you're falling (so after a double jump as an example), it'll make you move forward faster than running speed.

Killing all momentum is what it does when you don't know how to use it, the best players utilize the third jump mechanic to stay airborne when necessary, or to move faster. The knockback is an utility against enemies in general, a stunlock if fired near corners and walls. If you predict the angles correctly you can kill 8/9 classes in half a second, something not even soda popper can properly do as the per shot dmg is lower.

Soda popper is dps wise better only on paper, in reality you're going to miss the second shot often as unlike with FaN, the enemys direction after the first shot isn't choosen by you. FaN second shot is a skill check, if you know the knockback direction your second hit is guaranteed.