Arc skill mechanics

I have a question and I am quite confused about the mechanics of the Arc skill.

I would like to understand what causes Arc projectiles to fire 4–5 projectiles forward in a single direction. While playing, I sometimes see this behavior occur but I do not understand how to consistently trigger it.

My understanding is that this effect may come from the Wildshard gem. However, when I normally use Arc, it only fires 1 projectile forward and 8 projectiles around the character. I cannot get the 4–5 forward-firing projectiles that appear only in front of the character.

Could you please explain the mechanic behind this interaction and how it is triggered?

Thank you very much for answering