Chance to cast skills APPEAR to use mana when triggered, but in an inconsistent way - the mana orb will reduce, and if the display reaches low mana you’ll be unable to use skills, but moving your character will then revert the orb to the actual amount of mana you have.
This is easiest to duplicate with an amazon using Brand and Strafe with low mana - your orb will drop very rapidly from the CTC bone spear, but moving your character will result in the orb partially refilling. Can confirm it isn’t just strafe, as a Normal Attack will use mana as well as per the CTC Bone Spear.