I have Staff of the Chosen which does

50% Increase in Projectile Speed
118 Piercing Damage
354 Poison Damage over 3.0 Seconds
+1.5 Energy Regeneration per second
Grants skill: Summon Spriggling (Activated on attack)
20.0 Second(s) Recharge

Could someone explain to me the Activated on Attack effect? I havn't been able to get it to work yet.

Equip the staff and when attacking monstters with your left mouse button, there's a chance a Spriggling will spawn and attack your foes. Spriggling is a temporary pet. You can have up to three Sprigglings at any time.