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

A number of sources allow you to fire additional projectiles when using a skill besides Wildshards, such as the support Multishot https://poe2db.tw/us/Multishot_II. You can also get an augment for a chance to fire additional projectiles https://poe2db.tw/us/Thane_Girts_Rune_of_Wildness. The passive tree contains additional nodes that work similar to the Rune of Wildness, providing additional % chance that a skill fires additional projectiles (search "psychich fragmentation" in the passive tree or for the text "additional projectiles". That is how you can get arc to fire extra projectiles forward instead of in a circle.