Failnaught

Failnaught is a unique move in Byleth's kit, providing access to a slow but strong arrow, which can be turned around or canceled, and can be charged to become a long-range equivalent of a Falcon Punch.

(excerpt from Doze_Zoze's Beginner's Guide to Byleth)
FNC:

Another large part of neutral is Byleth's neutral special, Failnaught (FN). FN is a strong projectile with two different shots, a somewhat slow but strong arrow which we call FN1, and a slower but stronger laser beam we call FN2.

FN has the ability to cancel up until frame 60, meaning unless you charge beyond that point, you have a very low commitment on actually shooting. What makes this so good, is that FN1 releases on frame 45, meaning your opponent will never know if you'll shoot FN1 or begin charging FN2 then cancel the shot, unless you get predictable. This is known as Failnaught Cancelling (FNC)

Because they never know if you'll shoot them or not, they have to deal with the arrow. If they act too soon, however, you can easily cancel the shot and punish them for the option they chose. And if they stop dodging the arrow because you cancel it, you can just shoot them with it. This creates an excellent bait-and-punish situation that byleth heavily benefits from.