The Plasma Arrow is a special skill of the Cypher done by charging up plasma energy, resulting in a fast plasma bullet attack. Since Hiryu's Cypher can't shoot plasma normally, he requires the Scientist found in Kuramoto's Japanese residence to fix it in order to use the move.
The move is initiated by holding the Cypher up in position over Hiryu's head until it starts sparking with energy, after which a simple swing of the Cypher releases the projectile. The Arrow reaches a first charge after around 3 seconds, and a second full charge after 5 seconds. The first charge is roughly four times stronger than the basic Cypher attack, while the full charge is slightly stronger than that. Due to the charge time, however, it becomes difficult to use against moving enemies.
The Plasma Arrow is best used against Matic at the Red Dragon, as the projectile disarms him and makes him vulnerable to damage. While using projectile Tricks like "Fire", "Spark" and "SP-Ball" cause the same effect, the Plasma Arrow will not provoke Matic into disarming Hiryu if used near him, and since it uses no energy it can be repeated as many times as necessary.
Besides Hiryu, Dragon Fiend is also capable of shooting Plasma Arrows from his katana.